I mean, say we want to convince a certain charity to accept bitcoins.
We start collecting donations inside our community, for instance using someone's reputation on this forum (we have to chose someone with huge reputation since we need to trust that this guy will actually use the money for the donation).
Once we collect a fairly huge amount, we convert it into national currency, make the donation, and add the following remark : "this money was raised using bitcoins. Please consider using it."
Of course, the tough part is to find someone who will collect bitcoins. We need someone with a huge reputation. Maybe Satoshi ?
Concerning the choice of the organisation, I suggest I2P or Freenet, for they are imo the most likely to accept using bitcoins.