The @ ifdef statements only work during assembly, but that's AFTER the PBP program has been compiled.
So it can't directly affect PBP variables.

However, in ASM you can undefine bit variables and re-define them, which has pretty much the same effect.

Code:
Key       VAR PORTB
EnterKey  VAR BIT EXT :@ #undefine EnterKey 
StopKey   VAR BIT EXT :@ #undefine StopKey
StartKey  VAR BIT EXT :@ #undefine StartKey
ASM
  ifdef Devboard
    #define EnterKey  _Key,0
    #define StopKey   _Key,1
    #define StartKey  _Key,2
  else
    #define EnterKey  _Key,2
    #define StopKey   _Key,0
    #define StartKey  _Key,1
  endif
ENDASM