Using an overlay is how I'm going to want to do it. I can hook into org-ctrl-c-ctrl-c-hook. It means I can press C-c C-c to run the check.

I need to properly check that I'm inside a table and run this for all cells.

Then I probably need to hook into the alignment function to either redo the overlays or at least clear them.

This code will make the cell background red for the cell I'm in if the value is less than 1 or greater than 2 when I press C-c C-c ... But it is still buggy and will clear the overlays if one of them doesn't match the rules.