I hope.  I finally realized that we were going at it backwards: when
there are excess parentheses, they need to be treated as part of the
sub-SELECT, not as part of the surrounding expression.  Although either
choice yields an unambiguous grammar, only this way produces a grammar
that is LALR(1).  With the old approach we were guaranteed to fail on
either 'SELECT (((SELECT 2)) + 3)' or
'SELECT (((SELECT 2)) UNION SELECT 2)' depending on which way we
resolve the initial shift/reduce conflict.  With the new way, the same
reduction track can be followed in both cases until we have advanced
far enough to know whether we are done with the sub-SELECT or not.