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I've had an additional thought on how to make a Marseillais Chess ZRF more
optimized. Between each player's first and second move, have a dummy
player check whether either King is in check, placing a piece on a
specified location if either King is in check, and clearing the same space
if no King is in check. Then on the second move, each piece just verifies
that this space is empty before moving. This will eliminate a whole lot of
overhead caused by multiple checks of whether any King is in check.

It might also be useful to use two spaces instead of one. Checking both
spaces could be done with an or. Doing this would reduce a bit of
overhead. There could be two dummy players, a white dummy and a black
dummy. Each could first check for a marker indicating that it's side is
in check. If so, it would check whether it's still in check. If it was
empty, it would not have to check whether it's in check. In either case,
it would check whether it's side has placed the other side in check.

Another advantage of this would be the presence of visible check
indicators for each side. Zillions does not normally tell you when you're
in check. This would be a nice side effect of implementing the game in
this way.