For example, can they (or will they) revoke my degree?
I am surprised that you were issued a degree without having paid tuition for your last semester. In most cases I am familiar with, universities require students to be in good financial standing in order to receive a degree, for exactly this reason. If it was an intentional decision, not holding up your degree for nonpayment of tuition is unusually generous on their part. If it was a bureaucratic mistake: yes, it seems that in principle they could revoke -- or suspend pending payment, or whatever -- your degree. Given that you did not live up to your end of the bargain in a significant way, I would have to acknowledge their right to do so. Will they? I don't know: you could ask them.
Are they likely to pursue me for the money?
It is likely that you will be hearing from them within the next year, yes. When they contact you, you'll learn how serious they are and what their terms are. The chance that they will ask about it presumably becomes smaller as time passes, but you never know. In terms of how long you are legally liable for the debt: a good long time; ask a lawyer. My PhD program wrote to tell me that I had overpaid a bill in my final semester and they wanted to send me a refund check. I received this letter from them about eight years after receiving my PhD! (There was something in there about their not having been able to track me down, which I thought was pretty weird, but bureaucracies work in strange ways.) If a university was so durably interested in squaring an old account in my favor, I would not at all discount the possibility that N years down the line they could discover the issue and start it up again. Not doing anything would risk having this hang over you indefinitely.
Personally I have no moral qualms about not paying that last semester's fees, given that I've already paid upwards of $200k over the years.
Just because you have no moral qualms about something does not make your actions ethical. Many people, including future employers, upon learning about this would view it as a serious character flaw. If you plan on listing your degree on your resume then anyone who wants to can look into this at any time. Are you confident now that what you are doing five or ten or twenty years later is something so that having this information revealed would be no problem for you?
Added: The OP has modified his question. I will address those modifications.
First I want to address the point about the ethics. I read that carefully, and the claim that I ignored it is false. The OP's question was "What can happen to me if I do this?" and after addressing all of the specific questions he asked about that, I decided to address the part that there are practical consequences to behaving in a way that other people regard as unethical. Pointing out "If you do this, then you may -- or may not, depending upon the circles you run in -- get in trouble" is not an ethical argument in the slightest. It is quite the opposite. Only at the very end of my answer did I include two sentences which indicated that I viewed the actions as unethical. This point has been well made by many others by now, so I have removed those sentences. This leaves an answer which is 100% devoted to answering both the letter and spirit of the OP's question.
Now, on to the added points:
(1) This is an important detail, enough so that it probably should have been in the original question. Being charged for a full semester's tuition even though you didn't take any classes does seem unjust. In fact, it is so unjust that I think there is a good chance that it was a mistake of some sort, and the fact that you were allowed to graduate is nontrivial evidence of that (a circumstance which otherwise seems quite irregular). If you have received a bill for a full semester's worth of tuition, you should speak to some university official on the phone or in person and explain why that is not reasonable. Definitely mention that you've already received the degree.
(2) You are now explicitly making an ethical argument, so here we go. Referring to the nonpayment of a debt that you entered into voluntarily following a mistake on your part as "civil disobedience" is ridiculous almost to the point of being offensive. If you wanted to stand on principle, then you should not have agreed to pay the bill in the first place. You could then have asked them "Are you really going to deny me my degree after I've completed all the requirements and paid all the tuition just because of some missed graduation deadline? Because I refuse to pay another semester's tuition: that's outrageous. In exchange, I will tell the world that I do not have a degree from this institution but that I completed all the requirements and paid all the money for the semesters that I took." Instead you obtained your degree under a promise to pay and are now reneging on that promise. This is not a virtuous act.
(2 continued) You chose to pay more than $200,000 for your college education. (Statistically speaking, it is much more likely that your parents paid. But I'm taking you at your word.) Is that more than you should have paid? Well, that's for you to decide, but it's a hell of a lot more than I paid or would have been willing to pay. In fact I paid no tuition whatsoever, because I got a merit-based scholarship. My sister got into an Ivy League school but decided to attend an excellent state university instead and paid a fraction of the price. At my current institution, in-state residents who maintain a 3.0 GPA pay no tuition whatsoever. And so on. The flaw in your moral stance is that you willingly entered into this bargain. Arguably you shouldn't have. Not only would you have more moral high ground, you would probably have at least another $100,000.
(3) For an international student to come to the US and pay full tuition, it is likely that you come from a wealthy background and that you and/or your family have made a very conscious investment in getting a degree from an elite American institution. You could have attended a university in your native country, most likely for a small fraction of the cost. Since you did not do that, I am guessing that you want to be able to advertise your American degree. If that degree carries a lot of prestige in your home country, then it seems likely that they will not just take your word for it that you attended and graduated but will want to see official documents and/or transcripts coming from the university. Investing $200,000 and not having that to show for it would be a very poor outcome for you...practically speaking.