Nice, that is very helpful for those interested in writing an engine for it!
My musketeer engine is now available in NebiyuAlien and I was able to play a full game with it in my GUI.
The piece letters I use are similar to yours so it should be compatible with your engine, but note that
Stockfish uses a different piece letter for the Chancellor I think. So if we want to standardize it better do it now.
The variant name I use is "musketeer", and also I don't suffix gating moves with promotion letters.
One thing I do is to allow king captures ( I seem to get 2x larger nps with it) so you may see king captures in its PV.
Also, stalemate detection was kind of hard to get right with this approach, so I do it only the first two plies.
Without that, it was considering stalemate moves and was thinking it was winning ... But after the first two plies, it still assumes
stalemate to be a win, I don't think there is a lot of damage due to that.
With regard to NN, I am thinking of using existing NN to see how it does with the Archbishop and Dragon pieces selected. These are rook+knight
and queen+knight, which I plan to treat as two pieces on the same square and see if it gives reasonable outputs. For the rest of the pieces,
maybe an approximation will work, otherwise, a net has to be trained from scratch...
I am not yet able to make it play under xboard in "alien" mode as it seems it doesn't update the board
from the setup command that the engine sends. Gated pieces still remain on the back rank even when the piece
in front of them moves, so I guess xboard still constructs its own board from the moves engine's send (no alien support?).