I might have achieved something approaching what you want with:
This column definition ==>>
And this setting ==>>
The columns don't have a set of fixed pre-defined values, but I can use dot notation very effectively for categories that have a limited number of possible values, e.g. Mammals.Cats.Lion and Mammals.Cats.Tiger etc.
I use a similar column for Dates that have varying precision, such as: 1987, 1884.05, and 1995.07.29. I loathe having to conjure up false values.
BR