Hmm... it's a good idea, but what I'd really like is to get rid of those menus altogether in favour of choosing items and monsters from the terrain palette area. It would mean that you need to choose whether to show terrain, monsters, or items in that area, which means it may be complicated to implement since the BoE editor (unlike BoA) does not already have the capability to change the palette...
Even with that change, a "recent" or "favourites" list may be useful...