This "legal loophole" is trivial for journals to close.
In fact, I wouldn't be surprised if most journals hadn't, in effect, already pre-emptively closed it.
Basically, any publishing agreement vetted by a halfway competent lawyer will already have a clause requiring the author to assert that they in fact do have the authority to grant the journal the rights that the agreement says they do. For exclusive copyright transfers, this is likely to explicitly include an explicit or at least implicit assertion that the author has not granted any prior conflicting licenses to the article.
For example, here's a quote from a sample publishing agreement I found on the first page of Google results:
Author Representations/Ethics and Disclosure
I affirm [that...]
- Except as expressly set out in this Journal Publishing Agreement, the Article is not subject to any prior rights or licenses and, if my or any of my co-authors’ institution has a policy that might restrict my ability to grant exclusive rights under this Journal Publishing Agreement, a written waiver of that policy has been obtained.
If it turns out that the author has, in fact, previously granted a non-exclusive license to the article to a third party, in a way that is not covered by the explicit permissions described in the publishing agreement, then the author will have violated the publishing agreement, and may be considered to have negotiated in bad faith (since they presumably knew about the previous license when they signed the publishing agreement). What happens next may vary, but at least the journal has perfectly valid grounds to retract the paper if the publishing agreement is breached.
Of course, all this is negotiable.
If you do, in fact, want to both publish your paper in a traditional journal and also distribute it through alternative channels (like, say, arXiv, an institutional repository, your personal website, etc.), you can just tell the journal that you want to include an exception for that in the publishing agreement. You may even, if you like, grant such an alternative distribution license already before submitting your paper to the journal, in which case it's basically a "take it or leave it" deal for the journal — they can't force you to cancel the earlier license, but they can just decide that they don't want your paper.
In fact, many journals do already expressly permit, or at least implicitly tolerate, various forms of third-party article distribution. For example, the same publishing agreement I quoted above also explicitly permits "Scholarly Sharing", which is defined as:
Preprint: Sharing of Preprints by an author on any website or repository at any time. When the Article is accepted, the author is encouraged to include a link to the formal publication through the relevant DOI. The author can also update the preprint on arXiv or RePEc with the Accepted Manuscript.
Accepted Manuscript:
(i) immediately on acceptance: sharing of the Accepted Manuscript by an author:
- via the author’s non-commercial personal homepage or blog
- via the author’s research institute or institutional repository for Internal Institutional Use or as part of an invitation-only research collaboration work-group
- directly by providing copies to the author’s students or to research collaborators for their personal use
- for private scholarly sharing as part of an invitation-only work group on commercial sites with which the publisher has a hosting agreement
(ii) after the embargo period: an author may share the Accepted Manuscript via non-commercial hosting platforms (such as the author’s institutional repository) and via commercial sites with which the publisher has a hosting agreement.
Of course, journal publishers don't really grant such permissions just because they feel kind and altruistic — they do so because authors have actively insisted on such permissions, and because they judge that any potential loss of income from such non-commercial paper distribution will cost them less than the loss of manuscripts from boycotting authors and/or the PR cost of sending lawyers after respected academics who are just doing what they consider to be reasonable and customary sharing of scholarly information. The publishers know that they can only go so far in demanding exclusive distribution rights without risking a major backlash, and they're generally smart enough to adapt and make the best of the situation by explicitly releasing those rights that they couldn't practically enforce anyway, and making it look like benevolence on their part.
So, in short, there is no legal trick to "disable" distribution restrictions by publishers — at least not one that would work more than once per publisher. What does have a chance of working is politics, media and activism: publicizing the issue, exerting pressure on publishers, and voting with your feet. But, unless you happen to be a really respected figure in your field, the kind of person whose papers top-tier journals will fight over (or unless you happen to be in sole charge of a major institution's subscription policies), you probably can't do much of that alone. Certainly, having a CC license grant in your back pocket isn't going to be a silver bullet.