So, search as I might, I can't find anything about specifying recommended packages in a spec file.
I wouldn't even consider it possible if I hadn't seen it in the spec file for Mono 2.10.9. It reads the following:
Recommends: libgdiplus0 >= 2.6
But rpmbuild is definitely complaining about it. I'm guessing it's not something that's actually allowed. Is that the case? Is there another way to recommend some other packages during the install phase?