>The data is being parsed in and out of the application, so changing the data type would mean reprogramming three systems. If I can make the interface connect to the data as is then that would be preferrable.

Not sure if this will work:
Do not set ControlSource to that OptionGroup.
Add a method in the form named SetOptValue()
In it:

IF thisform.OptionGroup.Value # VAL(Table.Field)
   thisform.OptionGroup.Value = VAL(Table.Field)
   WITH EVALUATE([thisform.OptionGroup.Option]+ALLTRIM(STR(VAL(Table.Field),2))
        .SetFocus() && Just to change focus to the right Option Button
   ENDWITH
ENDIF

Then you should call that method in Init Event of the form and Everywhere you change the record pointer of that table.

If InterActiveChange event of the OptionGroup put:

REPLACE Field WITH STR(this.Value,2) IN Table

That is on top of my head and you should polish it to work for you in any occasions.