I ran into the same issues and we have not proceeded with it. The difficult part about it is that the realtor must invite the originator to advertise on their page. If the originator agrees, it lowers the originator's advertising bill. The more originators, the lower the ad bill for the reatlor. The realtor isn't getting less advertising. They are getting the same amount as before (webpage size is the same, their name prominence is the same, etc) but just for less. I just think the way in which it's setup is tricky. The realtor is getting a heavily reduced ad bill thanks to the originator, who is getting a small blurb at the bottom or side of the page (if I remember the formatting correctly). If the origiantor could buy the spot or blurb on their own, I think that would help - without the realtor's ad bill being reduced. Linking the two gives rise to implications.
I try to think of a full page newspaper ad analogy where each gets to pay for their half. In this case, the realtor has paid for the whole page but reserved three lines near the bottom. Each originator that pays gets to be on a reserved line at the bottom of the page. However, if three orignators sign up, the realtor only pays half price for the full page. I see it as a bit dangerous until there's a clear opinion concerning the zillow setup specifically or something similar like a webpage.
It was mentioned, I think by zillow or someone who had dealt with them previously, that some of the bigger banks have agreements with zillow directly for advertising. That was the next route we where looking at but management shifted to other priorities and that project fizzled.