I wonder what the legality of saying "this PR is available under the MIT license upon being merged into this repo" would be. A PR is copyrighted, and I guess it's just implied that a PR is available under the same license as the main repository, but an explicit PR license could override this (maybe?, IANAL).
I suppose they could merge it, copy it under the MIT license, and then remove it, which overall seems kind of silly, but it would at least get your PR merged. Plus, this trick would look really bad from a PR perspective.
GitHub's TOS include a clause that says you agree your PRs are under the same license as the project ("license in = license out"). I don't know what happens if you assert a specific license that contradicts that. It would be bad to do what you say, because if the PR is never merged, then third parties could never use the PR in forks. A better alternative would be to offer the patch under MIT or GPL at the maintainer's discretion, but that would create more confusion than it's worth.
After all, nobody is currently prevented from maintaining Gitea with the linked PR merged. The problem is that the major community maintainer is prevented from merging it due to a conflict of interest. License juggling won't help with that.
Your proposed "trick" does not seem to achieve anything - the code is still not present in the main ditribution, no?
My proposed "trick" was just to illustrate that it's futile to make a contribution MIT licensed for some people/purposes and not others. I think we're in agreement on this.
I suppose they could merge it, copy it under the MIT license, and then remove it, which overall seems kind of silly, but it would at least get your PR merged. Plus, this trick would look really bad from a PR perspective.