I'm not sure. I did a special event back in 06 and the manager for the event was the same woman who at the time was managing DFTW. (I'm sorry, the name escapes me.) I remember we were talking about weddings, I was interested in all the details from having worked them so often. It seems like she was telling me about locations that are not normally on the list, but again, this was in 06, so my memory may be poor.
She was very nice and it did seem like they could be flexible to an extent. I'm sure a lot of it would depend on where you want it. Resort locations might be easier than theme park locations. I remember doing a wedding at Sunset Point at the Poly, but that is not a listed venue. I don't know if it was listed in 06 and then taken off the list, or if it was soemthing they had requested. It never hurts to ask!