Hm.. Thanks again for your reply. I have several XBee Pro modules sitting around me but the only API commands I can seem to find in their manual are simple packetized send/receive commands.
I think that the bottom line is that the ZigBee is probably not right for my application, since I don't need the full duplex and all of the additional processing (support for various topologies, etc) anyway. I think that I'm going to try the seemingly simpler Aerocomm/Radiotronix/Radiometrix RF modems to build my own point-multipoint, unreliable, low-fixed-latency network.
Stevech, thanks so much for your advice. If anyone reads this and has modules that they've had positive experience with, suggestions are always more than welcome