That looks about right although I don't think you need the 3rd return since your rule would either return something from your picklist, or the contents of your custom field (if the picklist option for "custom value" is selected.
I don't know about EFI's capabilities, but if we were setting this up, I would go the extra measure of only displaying the custom text field on the portal when the "custom value" is selected in the pick list.
