Don’t display Unknown when using ProtoField with partial valuestring table

There is a field in my protocol that I would like to display as base.DEC_HEX, except when it is a reserved value, in which case I would like to display a string (with a dec or hex value in parentheses).

I do NOT want the values to show as Unknown (value), which is what happens when I create a string table with just the reserved value and reference it in the ProtoField.new call.

What is the best way to accomplish this? I know I can use an if statement and assign a label when calling tree:add() but this seems less than ideal, and would be a pain if I had several reserved values instead of just one.