Your variant poses the problem that both the King destination and the castling partner have multiple options. This requires specification of two squares. A possibility would be to let the empty square where the King should move to capture the Rook. (Or have the castling partner capture itself, when the King would end there.) Perhaps Greg has ideas of how to best solve this.
I don't really consider this a "castling" problem. What we have here is a rule where multiple combinations of pieces can move to multiple combinations of squares. There could be use in addressing this for any game might want to do this, but if we want to support this, I think it should be done in a general way, not forced into the context of castling. (So, I am not at all in favor of empty-space-captures-rook.) I think the robust way to address is as two separate moves, like "a1a2,b1b2". Doesn't the interactive diagram have a similar issue with the Forest Ox from Odin's Rune? It also must record a third square.
Also, if we're going to do an a1a2,b1b2 kind of thing, it makes sense to me to consider a separate delimiter for multi-move games such as Mersaillias. I think our life would be easier if "single moves" that consist of different parts are delimited differently than the separate moves of multi-move variants. For example, if a double move variant has, as it's first move, one of these moves that requires two different annotations, perhaps something like "a1a2,b1b2;c1c2"
In any event, this is a large can of worms, and I don't think we rush a poor solution just because this expiremental game has a crazy castling rule.
I don't really consider this a "castling" problem. What we have here is a rule where multiple combinations of pieces can move to multiple combinations of squares. There could be use in addressing this for any game might want to do this, but if we want to support this, I think it should be done in a general way, not forced into the context of castling. (So, I am not at all in favor of empty-space-captures-rook.) I think the robust way to address is as two separate moves, like "a1a2,b1b2". Doesn't the interactive diagram have a similar issue with the Forest Ox from Odin's Rune? It also must record a third square.
Also, if we're going to do an a1a2,b1b2 kind of thing, it makes sense to me to consider a separate delimiter for multi-move games such as Mersaillias. I think our life would be easier if "single moves" that consist of different parts are delimited differently than the separate moves of multi-move variants. For example, if a double move variant has, as it's first move, one of these moves that requires two different annotations, perhaps something like "a1a2,b1b2;c1c2"
In any event, this is a large can of worms, and I don't think we rush a poor solution just because this expiremental game has a crazy castling rule.