My vote is go for it. You are simply providing a service to find and monitor participants to wear their signature. Some people may be offended or think it's out of place, they can disable signature ads and move on. It's a product/service that is legal and free to people in most parts of the world if I'm not mistaken so it's not really out of place.
As for past examples the best I can think of was this campaign
Eroiy.io. It wasn't a site per say, but a coin dedicated to anonymous payments for Porn sites.
If anything would come up that they've been doing some shenanigans later on, could I get in trouble for marketing it here? Should I consider it in any way immoral to promote pornographic content here?
To answer these questions, I can only speak for myself and how I'd deal with it here on the forum. If they start doing Shady shit ( underage or non consensual videos, or theft) I would judge you only by how you handle things after you find out or it's brought to your attention. I have no doubt you would act appropriately in those cases so I don't have any worries there. The immoral question comes down to your own personal values, personally if they're here they have access to the internet and have been shown the way already.
I also can't see the forum having an issue, we've allowed at the very least cam girl promotions to be an ongoing thing. They likely still would be if not for a scammer being outed.