Ciao!
To the best of my knowledge no such thing exists and would be a bitch to code if at all. You would need to hire a programmer, give him all of your sponsor logins, etc. I don't see it a feasible or wise solution.
There are a few sites out there that list many sites' hosted galleries such as
http://sponsorhostedgalleries.com/ but they aren't neccesarily updated very often.
Your best bet is to get a gallery management script that can check for / reject duplicates and do things as you are now - by going to the sponsors' sites and importing their links.