I have forgotten how exactly I implemented this, but there is an variable 'brouhaha' which is set to an empty array in the include code. I suppose that setting it would work, and that the Play-Test Applet generates the code to set it, when you created it with Brouhaha squares. (At the bottom of the piece table there is a button that allows you to black out squares rather than place pieces. You should do that first, and then later move a piece to the blacked-out square to make it a Brouhaha square.)
[Edit] I checked it out, and the GAME code in the betza.txt include file simply tests in HandleMove whether the origin square is in the defined 'brouhaha' set and is evacuated (i.e. not also a drop, like in a swap move). If so, it deletes the cell as part of move making. (Which happens before testing whether the move was fully legal.)
It doesn't remove the Brouhaha square during testing for full legality for the purpose of highlighting. In theory this could cause highlighting of moves with pieces that are pinned on Brouhaha squares. (E.g. if c0, d0 and e0 are Brouhaha squares, and your King is on c0, your Bishop on d0 and an enemy Rook on e0, moves with the Bishop would still be highlighted.) It also doesn't remove the Brouhaha square when the piece on it is locust-captured. The latter should be easy to fix, if the need ever arises.
I have forgotten how exactly I implemented this, but there is an variable 'brouhaha' which is set to an empty array in the include code. I suppose that setting it would work, and that the Play-Test Applet generates the code to set it, when you created it with Brouhaha squares. (At the bottom of the piece table there is a button that allows you to black out squares rather than place pieces. You should do that first, and then later move a piece to the blacked-out square to make it a Brouhaha square.)
[Edit] I checked it out, and the GAME code in the betza.txt include file simply tests in HandleMove whether the origin square is in the defined 'brouhaha' set and is evacuated (i.e. not also a drop, like in a swap move). If so, it deletes the cell as part of move making. (Which happens before testing whether the move was fully legal.)
It doesn't remove the Brouhaha square during testing for full legality for the purpose of highlighting. In theory this could cause highlighting of moves with pieces that are pinned on Brouhaha squares. (E.g. if c0, d0 and e0 are Brouhaha squares, and your King is on c0, your Bishop on d0 and an enemy Rook on e0, moves with the Bishop would still be highlighted.) It also doesn't remove the Brouhaha square when the piece on it is locust-captured. The latter should be easy to fix, if the need ever arises.