Try this out. Won't work with RTI right now, but hopefully someday
http://www.networkthermostat.com/

One thing to note on the aprilaire, 2 systems can not controll it, so you can't have RTI control, and WAN control.

RTI's X-Panel will fix that, but who knows when it will be out.

The networkthermostat has 2 RS232 ports, so you could control it WAN from the remote software, and 3rd party when someone writes the drivers