I’m not sure if this is the correct place to post this, but I suggest that the Game Piece Palette section of the Reference Manual be updated to include a reference to the Place Marker trait, as a way for a module to “programmatically” place a marker from the palette.
Good idea - I have added the link to the source file for the reference manual. It should appear in the next version of VASSAL we release, and will work its way into the online version you linked.