It can certainly be done, but I think it would be very difficult if you did not have your own aircraft, and were not able to provide a decent amount of your own funding (not totally impossible however; I know of at least one guy who did a round the world flight and through sponsorship manged to acquire the brand-new plane, all the funds needed for the flight, and also money for charity!)
You'll also need to be ready for a lot of hostility from some in the flying community if you try and do something for charity off the back of an adventurous flying trip or event too, for some reason! I still haven't worked out why, logically - all I can think of is jealousy, or good old schadenfreunde. See the thread on here about the guy trying to break the "time aloft" world record, for a start.
However, there'll be a lot more people who think it's great, so don't be too disheartened by the nay-sayers! Having only 90 hours is not a total barrier to entry; I did a flight from Florida to California and back with just 150 hours, and even then people were telling me I was far too inexperienced (despite how easy US flying really is!)