You've hit on the essence of what I was trying to bring up.
The only thing is that an offside pass is NOT a foul by definition - it's an offside pass. The recovery of said pass by an inelgible player is what creates a foul.
I would personally prefer to see the spot of the pass beanbagged instead of flagged but either way - a flag can always be picked up
i.e. any B player can recover an offside pass, or an onside A player can recover the offside pass
You yanks wanna play Canadian ball now?