I am not sure you would be able to get a charity tourney online. This is something you would have to look into, but I have to say the chances are slim...basically because the site would have to agree to give some of its rake back to you, which may never happen, unless you are a VIP or the head of a nationally recognized charity. You would be better off having a freeplay tourney online at say PokerStars
and have someone hold the money personally.
I would do it, if the rake wasnt too high. The best sites to play on will ALWAYS be Poker Stars or 888
You are better off hosting a live game rather than online...just my opinion.