Some randome thoughts in no particular order -
Sounds like a good idea, raising money for charities is always a good thing.
You may want to contact Mike Kneebone at the Ironbutt Association - http://www.ironbutt.com/about/default.cfm
- to find out how others have done this, as well as to get any permission necessary to use the Iron Butt name to market/advertise the event. You may even want to check into sponsorship, if he does this type of thing (read on to see why).
Are you prepared to get the necessary insurance and such to sponsor such an event, or do you have a sponsoring organization (club, dealer, etc.) that carries insurance for such a thing? Having been involved with CCR for a little while as well as an officer and/or director of a HOG chapter (Harley Owners Group) for 7 years, this is way more important than you may think. As soon as you put something like ths together you are exposed to lawsuits and need to protect yourself. For example, someone goes down on your ride and they sue you for the vehicle damage and injuries. Without appropriate insurance you bear the burden of paying this if found liable in court, and they can take everything you have from your pocket change to your home and vehicles. Albeit not for profit and for a very good cause, you are still entering into a business venture that bears liability, and it should be entered into in a way to protect the principles from losing their shirts in a situation like this.
Speaking of non-profit, do you (or the sponsoring entity) have non-profit status? With money changing hands for entry fees, pledges, donations, etc., there needs to be some accounting of the funds and their dispersement. Otherwise you and/or others could get audited and saddled with an unexpected (and hefty) tax burden. Again, this is that business venture thing coming into play.
Don't let these comments scare you away from doing this; I think it's a good idea. You just have to remember that not everyone that comes out for a good cause will remain in that mindset if something goes wrong, and someone will eventually ask to see the numbers and wonder where the money went. Just be thoughtful and protective of yourself and your personal interests so they can remain intact after the good deed is done.