Ability to use facebook comment box with custom domain.

It would be awesome if we could add facebook connect's comment box functionality to a tumblelog. It looks like most of what we would need is already in place, with the exception of the cross-domain receiver file. since when using a custom domain, we obviously cannot add this file ourselves, it would be awesome if there was a standard xd_receiver.htm file hosted on tumblr that we could point to. would that even be feasable? thanks!