King Reza, Fencer: Well, I suppose whoever wants to share a Black Rook can always go to the PawnRegistry and ask for a Rook membership explaining that they want explicitly to share a Black membership.
If another person is interested in sharing (or paying for it all together) can always go there first looking for black rook candidates.
Unless, of course, Rooks can't place an entry in the Pawn Registry (now, that would be a first - a feature not available for Rooks, lol)