Nice bump, I hadn't noticed this thread
When I first discovered Glest I was toying with the idea of a kind of summoner faction, I decided in the end it would be too much like magic, and while I hope I can still contribute a few models here and there, It doesn't look like I'll be making any entire mods of my own soon
One idea I had that might be useful to you as a 'training building' for some of these creatures (maybe the bigger stuff?) is a summoning ring (or sphere, perhaps), summoners could summon 'small stuff' like they do now, but bigger creatures would have to be summoned at the ring. Of course, I was envisaging creatures of a more 'fantastic' nature for this, I not so sure it makes sense to 'summon' and Minotaur for example.