If we already
had the unique login system, it would only be a fact of
<form
action changing at the user level.
No. The idea is to handle the upload from the home wiki, so to speak;
the user shouldn't ever need to be directed somewhere on Commons. And
this particular aspect has nothing to do with single sign-on (although
it's needed before we can go ahead with it, since images must be
associated with an uploader).
I understood the idea. But instead of doing it at the base, we could do it
on the top. It could then me more or less transparent but wouldn't matter
too much. And both methods need the single sign-on (or at least some kind of
identifying as user@project), so i wouldn't say it has nothing to do with
single sign-on, as it's a blocker.