Thanks for the mounting suggestions, but I think installing on the house is just not practical. First, I have a hip style roof on the main part of the house with about an 18" overhang, and I'd still need a pretty lengthy mast to get near the ridge elevation given the roof slope. And second, I'm an ex-HOA Board member and currently on the Design Review Committee for new homes and exterior improvements, and legally challenging the rules for this doesn't seem prudent.
As for antennas, I had already found a few yagi's out there for this frequency range, including the one WA4OPQ found for $30 on eBay (my guess is that's a call sign?). Getting some gain by going directional would be the only rational approach, I think, as an external dipole will continue to throw energy everywhere it isn't needed. Adding an antenna on the receiving end would provide some help, too, but it would be nowhere near as good as directing the transmission energy.
I'm an electronics tinkerer, not a licensed radio operator, but I'm handy with a soldering iron. What I don't know is how to best handle the short run to a connector at 915 MHz... maybe in similar fashion to the wifi connections in a laptop. Since it appears I'll be the guinea pig for this, any expertise out there with wire type (mini coax?) and configuration so as to keep the little transmitter happy would be appreciated!