I forgot to mention this yesterday. I've set up a github project (see http://github.com/davorg/rpm_stuff) where I'll dump bits and pieces that I'm writing to make my RPM-building life easier.
The first utility I've uploaded there is called can_rpmbuild. You pass it an RPM spec file and it tells you which of the dependencies are available from the RPM repositories that you use and which of them you'll have build yourself.
I'm sure it can be improved. Patches welcome of course. And please let me know if you find it useful.