How about an option for zquest (In the Misc-->Options Menu) that you can check that will make it so something will appear & flash over combos that require an undercombo but where the undercombo has not yet been set?
Perhaps it would also check instances like where you have a slash combo & flag 16 over it--because that's exceptable, the icon would not appear. However, if neither a flag has been set nor undercombo, then a small square would appear over such combos (just like the small "Show Misalign" triangles appear on the sides for walkability misalignments.)
Maybe such options are annoying, with all the flashing things and stuff and all the exceptions, but it would be nice when done with the quest and when you are "debugging" it, trying to seek out where you may have forgotten to set the undercombo, instead of always going to each screen and repeatedly click "Undercombo" in the data menu over & over again.