I'm not sure if a true partnership is what you would want. Unless your ok with giving up control of how things are run. Thats just looking at it from a legal issues as to what partnerships are about. It may be both simpler and more complex from a legal standpoint, but that can be looked at, and contract law issues researched.
I think offering a place to host a dedicated subforum where somebody could control content within cetain rules of the parent froum could be a good thing for all parties. You could lease forum space to them on a monthly basis. This would be much faster and simpler for people who have a product or a forum speciality that they want to mod over. They would have most all control except for having to stay within certain ratings guidelines, (no porn, ect).
I don't think it it's a bad idea, and it could work in all parties best interest. It needs to be researched and a format, and contractual issues need to be worked out.