well it's already possible: you can select play from piece set and set up a board as you wish!
all these games are unrated.
but I think it should be not too difficult to implement some variants.
BTW: I think Russ and Chrismo like chess variants: they implemented some of them to their chessengine Rival.
th
@rus: you could add "FRC" in the play piece set in the preset board (reset, kings gambit, sicilian defence, Fisher Random Chess). (and a option: random colour)
th