With the old version on nnimap, I had a hack in place that allowed
article editing; but it wasn't ready for others, because it didn't
update the summary buffer correctly or preserve marks.

It seems to me that this shouldn't be such a hard thing to support
properly, though.  Gnus can already move an article between groups, and
editing an article in a backend that has immutable articles is like
moving it to the same group, except that the article is changed before
being saved to the group.

So I think it should be possible to re-use the code that is invoked when
the user types `B m'.  An extra argument (or a global variable) could
be used to tell nnimap-request-move-article that the move isn't
to be considered internal and to tell nnimap-request-accept-buffer
to take the article from the editing buffer.  That already would be
a big step forward, but even better would be to update the summary
buffer to replace the deleted article with the newly created article.

Here's a crude patch against the old nnimap that illustrates the idea,
but which doesn't work.  But maybe something similar will work with the
new nnimap (and maybe it would only be 10 or 20 lines of code)?

(And maybe this can even be done independent of the backend, just
assuming that the backend can create articles?)

Dan