Should I consider it in any way immoral to promote pornographic content here?
Imho as long as the already discussed parts (NSFW marking near the links, no porn in avatars or signatures..) it should be fine.
If anything would come up that they've been doing some shennanigans later on, could I get in trouble for marketig it here?
Well.. this is the same for all signature campaigns.
A good campaign manager need to do a bit of background check to make sure he doesn't endorse scams. And always be prepared to stop the campaign (and reimburse participants) if anything shady comes to surface.
If you don't do this then you can have troubles indeed. You know as good as me that the ones enrolling for a signature campaign don't do the background checks (some may simply rely on you!) and if anything is bad, they'll blame you.
As I said, this is no different from non-porn campaigns, after all. So no extra worries.