Maybe we can learn a few things from the
DOSBox pledge which was sort of similar but on a much smaller scale. That was essentially a one-man crew with a hobby project and without any real legal obligation, and the persuasion process was far from "100 BTC you say? Of course I'll accept bitcoins now!".
They accepted it though, and the address is still there
I certainly wouldn't stand in front of the salvation army guys and say I've got money here but only if you use "this" currency.
Agreed, that sounds silly. How about we say: "There are people who want to donate to your cause, but don't want to have part of their money eaten by paypal and such. Bitcoin donations would allow them to donate arbitrary amounts, large or small, without creditcards or fees. We've already collected some money in Bitcoins for you guys, can you set up a Bitcoin address so future donations could go directly to you?"