K.N.
I understand your intention, however I'm not sure I would involve "Santa" in being scolded for not taking care of last year's toys. For starters, it would be a bummer on Christmas morning to be told essentially 'You could have gotten more toys but you didn't deserve them... If only you appreciated your toys more... You were good but not good enough.'
Secondly, what potential message does this set you up for next year? I mean, let's pretend he immediately stops abusing his toys during 2011--He will expect the motherload next Christmas--But if you can't afford it, then the message is reiterated in his mind again: You were good for a few toys, but not good enough.
I think you should preserve Christmas morning as being a 100% feel good moment. As far as presents go, give what you can afford. But you shouldn't mask budget strain with a personalized letter from Santa. That is not going to make a happy morning for a 5 yo.