I doubt there exists something like 'official rules' for this game. I don't imagine much thinking went into the design of it, and it is actually likely that the designer was not even aware of the en-passant rule (as most people that know Chess do not know this rule).
It is a moot point anyway; this game can be won in a straightforward way by the full army without ever having any of his pieces captured, e.p. or otherwise. You just leave all your pieces protected or out of Maharaja range at any time, while developing to eventually cover the entire board. A good start is pushing your edge Pawns untill these are Queens. And then, under cover of those Queens (which stay on the edge to keep the Rooks protected) push the b- and g-Pawns promote these too, etc.
You would never even be tempted to double-push a Pawn if the square it passed over would not be covered by another Pawn, and thus be taboo for the Maharadja.
I just heard about the chess variant "Maharajah and the Sepoys", in which black is a standard chess set and white has only 1 piece, the "Maharajah". I was wondering which is the "officially correct" explanation of the Maharajah. I have heard that it can move as any other chess piece, and I have heard that it can move as either a queen or knight. On first pass the two definitions sound equivalent, but can the Maharajah capture en passant? Note that a queen could capture a pawn that has moved 2 spaces, but it would occupy the pawns square, which is a different outcome than if a real pawn had captured en passant.
Thanks for any help! I understand that being a variant, you could play either way as long as both players agree, but I'm curious to know the official ruling on this.