If they're serious they should dump the cash into a trust controlled by people outside BFL. Or they should make sure to give a few BIG chunks of that cash out immediately to worthwhile causes like zerocoin. Charity in my mind was like cancer treatment or hospitals or something...
+1 This.
They should use an escrow to prove they have 1000 BTC to donate and have that escrow donate it on behalf of them publicly so there is no funny business.
John could do it.
Likely BFL doesn't want to do this because it involves an honest 3rd party to keep things transparent.