The name of the game is now Pocket Mutation Chess. I've dropped the whole
demotion idea. Now any piece retains its value when pocketed and can
optionally mutate into another piece of the same value class. Promotion
is open to all pieces except the highest value piece and the King. This is
done by pocketing the piece when it is on the enemy's back rank--the piece
mutates into a piece of the next higher value class.
You can keep a piece in your pocket as long as desired, but can only have
one piece in your pocket at a time.
Stronger pieces are more restricted in where they can be dropped when
leaving the pocket.
You can pocket your King, even in check--but in this case, you must drop
it on the next turn if there is any empty, unattacked square on the
board.
This makes checkmate of a player with an empty pocket impossible, so bare
King also wins (immediately, no chance to draw by baring the enemy King).
Stalemate is virtually impossible, but is a draw if it occurs.
I'm still testing this but it seems playable. I have a ZRF which I can
email to anyone who would like to test the game before I submit it.