After clicking the relevant square (the to-square of a click-click promotion move, or the right-clicked square where you want to put a piece in Edit Position), you don't pop up anything, but simply display a piece in that square. As long as the mouse button stays down, you can then move the mouse to alter the identity of that piece. When you see the piece you want, you release the button, and voila! XBoard either executes a promotion to that piece, or creates it there.
I just pushed two patches to the http://hgm.nubati.net repository, popups branch, which implement this. (For XBoard; for now it might break WinBoard). One for the piece menu, one for the promotion popup. It can be tested through
xboard -sweepPromotions true -pieceMenu false
(These are persistent options, so you would have to specify them only once.) On drag-drop moves the promotion popup still appears, but if the new method is considered convenient, I might treat those moves in the future as "alwaysPromoteToQueen". How often are under-promotions needed anyway? Offering users the click-click option to perform them seems good enough. (To me. But I don't play...)
Please let me know what you think about this!