More good news from lwlib-Xm.c. This file used an XmNunmapCallback instead of an XmNpopdownCallback in the menu bar because the latter didn't work with some LessTif version. With current LessTif, XmNpopdownCallback works like in Motif as far as Emacs is concerned. There's a slight difference which doesn't affect Emacs, but might be interesting: When the the callback function is called, its 3rd arg CALL_DATA is always null in LessTif, while it is set to something non-null in Motif. I couldn't find out what that argument is supposed to contain, though.
