I'm sorry, but I'm not aware of any sensibility option for Keyspan receivers. I'm afraid what you're looking for wouldn't work even if Remote Buddy supported such an option, though, for the following reasons:

1) the Keyspan receiver will wake to *any* IR signal (sometimes even to direct sunlight) provided that the signal persists long enough. Even if that time span could be increased, you'd still end up sending a long-enough signal when keeping a button pressed on your Apple Remote.

2) support for PPC has been discontinued in both Remote Buddy and it's driver (Candelair) for some time, now, so updates wouldn't benefit you

There's a chance this could solve your problem, though: 
The Manta TR-1 also supports waking up your Mac - but only with a Windows MCE remote control's power button. It's smart enough to decode IR input even when asleep and only issue a wake-up signal over USB when receiving aforementioned IR code.

Best regards, 
Felix Schwarz