>As for special threads like the ones you linked, it's tricky to put to words as policy without confusing people, but if it would be helpful, I can try to put a note about possible exceptions/edge cases somehow

I just got an idea, that might solve this with very little effort.

we do not want a lot of threads in conflict with the rules popping up all over the place,
my "special threads" suggestion could have that result and was not a good idea.
on the other hand, from time to time such threads do get created so there is a certain demand to do that
and sometimes for good reason, which is why exceptions are made.

as we previously discussed, /req is a weird grey area, where apart from something real anything goes, as long as it is a request.
furthermore you said, that it is not a problem, if a request turns into a real thread an becomes big. there is precedent for that on /req already.

with a single rule that is more of an instruction really, this fact could come in handy:

if you need to create a thread that crosses between boards, you can do that by creating a permission request (PR) -thread on /req.

1) the title has to be PR: + the kind of crossing you are asking permission for + the content.
e.g.: [h]PR: /ai /3dcg (artist)[/h]

2) the first post must explain, why you see the need to create such a thread.
e.g.: I want to start a collection of an artist who works with both, ai and 3dcg.

another example for the thread already existing on /g:

PR: /g /s (death-expulsions)
I want to post pictures of people pissing, shitting and vomiting their guts out only while dying, please add more if you can.

and so forth.

those threads could stay on /req forever. everyone could immediately recognize by the title, that this thread has administrative approval,
or is at least waiting for it and the title also functions as a warning (attention! contains scat...)

it would also be possible, to move the already existing problematic threads to /req and just leave a comment in the old place because of bookmarks.
cleaning up the board. that would also mean, there are no exceptions anymore at all, which makes things easier and prevents discussions.
something that doesn't belong gets deleted, or if it isn't garbage, moved to /req. period.

and on /req itself, the special titles set those threads clearly apart from all other requests and are easily searchable.

all that is already possible under the current rules without changing a thing, it only needs to be a request.
apart from the artist threads, where they post their own stuff, almost all threads are requests anyway.
you post something and implicit ask for moar. "hey let's all post this & that" is actually a request too.
and since /req contains the least restrictions, almost half of GURochan, maybe more, could technically be on /req.

why ask for permission, if you can do that there anyway, according to the rules?
because you need to ask for something, to make it a request, otherwise it wouldn't be "legal", which would especially prevent artists from posting
their own stuff, since they don't ask for anything. so, why not simply ask for permission?

another cool thing is, that no admin has to actually grant permission, ever. there is no work in it.

in conclusion, the whole thing with exceptions from the rules could be dealt with, by adding a little instruction
and maybe later doing some cleanup, given time.