As I imagine "Perfect bitcoin wall", linux is a good choice. By linux I understand bare minimum for console bitcoin client.
Linux core + bare minimum to run bitcoin + firewall that blocks everything except bitcoin client. LiveCD or USB boot, or VM(heavily encrypted HD image, encrypt ram if possible).
Password to log in, pin for every action with bitcoin.
Couple of scripts:
Send N bitcoins to hardcoded spending wallet(password to change spending wallet address)
Move bitcoins to a new wallet. Maybe send bitcoins to the same wallet but new address(will it work and do the trick?)
Maybe simple unicode GUI.

Almost impenetrable by conventional means. Especially if done without VM.