but it doesn't work well.
That's for sure... it's horrible.
Basically, the idea is that the driver causes the various pieces to appear in the proper places on the chessboard. Using the system mouse, one can click a piece (to pick it up), then click where it should move to. The driver then makes the pieces move as needed. If the artwork has buttons, the mouse can activate them too.
The situation in MAME (last time I looked) is that a bare board is shown (just the artwork, basically), and nothing can be done. Entirely useless for users. The LUA-enabled ones sort of work, but you soon lose synchonisation between the board and the driver, rendering it useless again.
Before mboard.c existed, the code was in each driver, modified to suit that particular situation. If we were to go back to then, the perceived license situation might be able to be worked around.
From what I know, the fhub hack reinstates the old behaviour with tweaks, thus making the games fully playable. Unfortunately, this causes the issues already discussed, compounded by refusal to release the current source. There's not much point speculating further while this situation exists.
As for MESSUI, all questions must be asked on the official support forum, not here.