Great idea Lichen, here are some ideas to get the ball rolling....
Make it a nationwide one-day tree climbing event held at the various groves around the country (and internationally if there is interest).
Set up three levels of flags in a tree, each level reperesents a donation level. The goal is to touch the flag to earn the right to donate. Matching funds per flag touch could be arranged with corporate, business or other sponsors. Individual climbers could get donation pledges from friends which would allow the climber to "pay" for each flag they touch. The more pledges they get the more flags they can try for.
For the first level it would be a couple bucks per flag touch and there would be multiple flags reachable by first time facilitated climbers.
Level two would be a little tougher to get to and there would be less flags available, reachable by climbers with the ability to repitch. The donation amount is increased for the intermediate level flags.
Level three would be very challenging and would be a substantial amount donated per touch. Maybe only 3 flags available. A climber would have to be skilled and have raised enough in pledge money to touch the top level flags.
Any climber is free to donate their own money to their pledge cache of course.
There is no emphasis on speed to reach a flag, if you take all day to reach the toughest flag then you've earned the right to turn in pledge money and hopefully get matching funds from sponsors.
Feel free to revise, refine or reinvent, this is just to get things started
-moss