bigolbug:
I plan to start by using a FKS method of turning one frequency on and off.
Turning one frequency on/off would normally be referred to as "on-off keying" or OOK. FSK is "frequency shift keying" which would involve multiple frequencies.
For tools to help debug or diagnose encoding, I would recommend a logic analyzer -- something simple like a bus pirate would be fine.
http://dangerousprototypes.com/2009/11/03/bus-pirate-logic-analyzer-mode/
If you will modulate plain async serial onto the signal, either a bus pirate or Sump will have a usable decoder.
Although I would recommend using VirtiualWire as an excellently suited encoding scheme
I do not believe there are any pre-built logic analyzer tools for debugging a virtual wire link.
http://www.pjrc.com/teensy/td_libs_VirtualWire.html