At what point in this process, and by what mechanism, does a patch become a
"GPLv2 patch" or a "GPLv3 patch". I'd argue that the patch itself has no
such status at all: as of the time it's posted, its copyright is owned by
the FSF, but that's all that's happened. The assignment agreement
obligates the FSF that all "distribution" of the patch should be under
GPL-like terms, but it's completely unclear as to at what point those terms
apply. For one thing, there are multiple possible licenses even ignoring
the v2 vs. v3 issue: GPL, LGPL, GPL+exception, etc.
So if you see a patch posted on an FSF mailing list, what copyright license
status does that patch have? My feeling is that as a practical matter, it's
irrelevant since the patch is a derived work from some file and the license
that applies to that file trumps any that might be viewed by some mechanism
as applying to the patch in isolation. If I'm patching an LGPLv3 file,
then my patch is a derived work from that file and so is also LGPLv3.
End of story.
Now, suppose I apply it to the GPLv2 version of the file. One could argue
that such file is now GPLv3 and I think that'd be correct. But since the
parts of the file being patched are identical, the patch is indistinguishable
from one that's derived from GPLv2 text. This strikes me as a VERY murky
legal areas.