Foreign Key constraint on fixed value field - Ever appropriate?
The proper thing to do now would probably to make one table for each subgroup ...


Probably.


  It is though very unlikely that I ever further need to refer to the subgroups...


You probably said something very similar when you made that first questionable design decision.  There's a reason that normalization never asks the question, "How often will I need to refer to whatever?"

If you don't enjoy fixing this kind of problem over and over, do it right today and get on with more useful and fun work.