I posted something about this a while back myself. Since then I've done a bit of work on it, but it's by no means complete.
First of all, rather than trying to figure out average heights of cities, I've been using altitude above MSL (mean sea level), which is something that I would imagine would make determining this stuff easier by using pilots' maps. In contrast, AGL (above ground level) seems more like what you're saying.
Second, you're going to run into problems when you try to visit some of the really tall radio & tv antennae out there. From my research, the "alternate" DE HP would be the WBOC tower (read: "antenna") at about 1,002' MSL, as compared to Ebright Azimuth's 448.' At 2,407,' Alabama's Cheaha Peak doesn't even retain it's crown when compared to the WTTO tower, which reaches about 2,656' MSL. The best you could do in these cases (legally, I suppose I should add) is visit the base & look up at a really tall antenna.
A couple of resources I've used (in addition to my MapSource software to get ideas of ground elevation) are:
Unfortunately I can't find the link right now to the radio tower info, but I'm pretty sure it was either contained in the above sites or linked from there.
E-mail me off-list if you want to combine efforts on this.