Seems so yes.
Did some more digging and found that all possible shortcut combinations are defined by order_gui.cpp on source code level (https://docs.openttd.org/order__gui_8cpp.html
For now gotta still use that drop down menu to select needed refit option for trains.
Looking through order_gui.cpp code, it will need some additional coding, but is probably doable.
For now can only hope, that maybe in some future openttd release on someday, developers will add "refit to available cargo" to possible hotkey combinations.