just hold a charity race - where all the entry fees and whatnot are donated to charity.
ask around and get some people/companies/RC companies to donate some prizes to use for raffles...do a 50/50 - where 1/2 the $$ raised goes towards the charity and 1/2 goes to the winner
there is a group of guys in Maryland (USA) that has done something like this for ALS the last few years and they've raise around $4000 for the event..you got an drawing ticket with your entry fee, and they you could purchase more - plus they also had 'high-end' prizes that tickets were a little more for these items were like autographed racer/sports items
check out here -
http://mini-zracer.com/forums/forumdisplay.php?f=101
and you can see some of the things they offered and maybe get some ideas
another track near me did a fund raiser for JDRF and raise almost $2k the time they did it..
racers will support it - and it should be a kind of easy way to raise a few bucks for the charity