I am a professor at a state school in the US, and American professors are currently the targets of a lot of undeserved grief (e.g. it is currently fashionable to believe that we are "lazy" and work much less than a full "business week"; that is not only a mistake but likely a dishonest, politically motivated mistake). So keep that preamble in mind when I say:
This is one of the most unambiguously unacceptable behaviors from a course instructor that I have heard.
Let me try to count the ways (but there are so many that I may get tired before I finish):
1) One of the most basic understandings between the student, the instructor and the university is the duration of the course. By a certain extremely public, agreed upon, well-publicized deadline, all of the coursework must be complete, and by a slightly later date that your instructor has been carefully informed of, the course grades are due. Planning to defer submission of the course grade after this date is just not the way the American academic system works.
2) Giving a grade of "incomplete" is not withholding a grade: it is assigning a certain kind of unsatisfactory grade. [Once in college a rather elderly professor literally did not submit any of the grades for the course I was taking. It was a little strange -- in these days we actually received transcripts in the mail over the break so the inquiry was less immediate than it would now be -- but he apologized and submitted the grades very soon after the start of the next quarter. That's withholding a grade.] In order to assign an incomplete, the student must not have completed the official coursework.
3) The instructor is not suitably respectful of the obvious problem which can occur if you assign students' grades too late: you may prevent them from graduating. Many American universities have policies to expedite final coursework and grading for (would-be) graduating students. Moreover, in the universities I've been involved with, beyond the written rules there is an unwritten culture that as an instructor you should think twice about any course practice that interferes with a student's timely graduation. (Sometimes it turns out the student fails your course and therefore does not graduate on time. That's "okay", but most of instructors would indeed think more carefully about assigning a failing grade under these circumstances and feel honorbound to convey the failing grade to the student sooner rather than later. What if their parents show up in town only to learn that the student did not actually graduate?) This practice is not hard to understand: one of very few commonalities among American academics is that we were all students at one time, so we should have some sympathies for the student perspective.
4) This course requirement is unusual and potentially problematic, and it is not listed on the course syllabus. I think that the whole point of the course syllabus is so that instructors cannot totally change their course requirements / grading schemes in the middle of the course, and that seems to be what is happening here.
5) Teaching a university course is not like playing a game of Truth-or-Dare: it does not give you authority to compel the student outside of the classroom and the dorm room / study carrel. Submitting a paper to a conference is a real-world action with real-world ramifications (the OP is rightly aware and concerned about this). It is creating work -- possibly rather pointless and frivolous work -- for other busy academics and/or professionals. It is setting a student up for harsh critique even up to the point of ridicule. It is also bullying a student into publishing something they didn't actually want to publish.
Okay, I got tired. What should a student do in this situation?
Talk to the faculty member in question.
This is an in-person conversation. You should think carefully both about want you want to say and how to behave in such a conversation. Your goal is to convey the specific problems and hardships this course policy will impose on you. (You don't want to frame it as an ethical or hypothetical discussion.) Be calm but very specific. Bring in a copy of the course syllabus and refer to it at some point during the discussion. Bring in a copy of an academic document that says you need to have your grades by a certain date in order to graduate. Say specifically: "I'm concerned that this course policy will jeopardize my graduation. Can we address this?" If your instructor says "Don't worry about it right now" then explain why you are worried about it right now. Talk about your family's travel plans, talk about the financial implications of having to enroll for another semester....
(The point of the above strategy is this: I imagine that your instructor likes this course idea from his perspective. Clearly he has not taken the time to think about it from your perspective. If you make him see the negative consequences, he is much more likely to repent.)
You should not be brushed off in your meeting: this is a serious matter. If in the course of the meeting you don't see things working out to your satisfaction, you should let him know that you intend to talk to (e.g.) the department chair about it.
By the way, apparently this took place almost a year ago. What happened??