OK, in the 30 min or so since I originally posted, I did more research OS and the RPi platform and my thinking is as follows:
1. Implement the 2-wire interface as a GPIO plug in module for the RPi.
2. Optionally implement a bidirectional valve driver interface. 3 wires would be easy (common, 24VAC/xmit, receive). Would make valve driver diagnostics easy and also support sensor networking.
3. Look at automotive single chip solutions for distributed power control. The networked vale problem is similar to the distributed automotive “switches and loads” problem. Perhaps there is a single chip solution out there.