As you suspected, Tim, it takes a little work to get this one to parse, but nothing too onerous. I had to look ahead two additional tokens to see if we had a refmod without a subscript. Failing that, if we're still looking at an open paren, we must have a subscript, which may still, optionally, be followed by a refmod. Otherwise, we have neither, and the null rule succeeds.
Parenthesized things followed by question marks are syntactic predicates.
Pccts parses these without affecting the current position in the token stream and suppresses any actions that might be associated with the parse. Both terminals and non-terminals may be used.