Quoting myself. What if, instead of possibly complex UI needs etc.. to make this happen, tray was also a button that we could drag to any other tray as we wish. Similarly to what eg. WoW is doing with forms and stances.
That way I could put combat tray to eg. button '4' on my harvest tray. Then on button '4' I had my opener in combat tray and the switch from harvest tray to combat tray and to do attack opener would be just hitting 4 twice.
Then if we so choose we could not bind global tray switch to anything, or maybe bind just harvest tray and other trays were icons in there as harvest tray has room for those extra icons anyway? This would free many hotkeys to do useful things when in that tray.