I don't know if anyone's thought of this but have you considered offering members the options of buying & giving a square as a chritmas present to someone else?
I'm thinking, the buyer donates to charity, buys a square as a gift which comes with a Onemillionmasterpiece christmas gift ecard with instructions on how to make the gift square live. I would suggest in the interest of not having more black squares, the system work on the basis that the recipient has to log in to make the square live.
Just a thought - I think I'd give squares if the option was there.