I have actually, alongside my work of compressing moves. My compression scheme averages about 150 bits per position, or about 35% of the standard text notation in EPD format.

The thing I optimized for is that there’s very often repeated blank spaces or repeated pawns on the same rank.

Also instead of storing castling status separately, it’s stored as a separate piece on the appropriate rook.

These take advantage of the fact that there’s 6 pieces, so 3 bits to encode them leaves two options remaining. One is PawnRepeating and the other is RookCastleAvailable, in my scheme.

There’s probably improvements to be made. I’ll write a post on it when it’s finalized.