Permitting this facility would probably require more effort that just getting rid of the legality check. Even though RHP doesn't actually choose moves itself, it needs a chess engine which understands enough to know stuff like when your move is legal, and when the game is won or drawn or a draw can be claimed. This is a non-trivial coding exercise. Probably, the code will be written to accommodate legal positions - for example, it will allocate enough storage to accommodate 16 pieces each, but probably not more. Of course this could be changed, but anyone who has ever worked in programming knows that changing things like this usually has far more knock on implications than you first expect. So it could be done, but it would probably be a lot of effort so you would need to justify this by demonstrating widespread demand.