I think anything for a full valid charity deserves as much support as you can give. If, as in this case, it means a slight breach of your guidelines, then as long as it is made clear that it is only for charity, I personally cannot see how anyone could be too upset about it.
I would suggest that perhaps you would wish to make an entirely seperate sub-board on the forum for this, which can be hidden when finished.
I would also like to say, that I think this is a very nice gesture on behalf of John Mathews, Martin Short and Bigger Picture.
Thanks guys, however this works out.