Agree! It is partly possible already but you need to have at least Legendary rank, better to be a staff member with green trust.
I can actually enroll in some of the known signature campaigns available here in this forum but the thing is, I'm not a fan of changing/updating signatures frequently and checking out spreadsheets to track my progress in a campaign. And also, I think having a Legendary rank partly helps in these kind of services, seeing that other member ranks can do the same advertisement too.