The current metagrammar parser forbits a sequence with only a single labeled element, as noted by @jbrains in issue 4 of @nathansobo 's branch of treetop. Digging in, it looked like this is because labels are only supported in sequences, and a sequence is distinguished by the presence of at least two expressions. This patch distinguishes a sequence consisting of a single expression with a mandatory label.
Support for sequence of single labeled expression
Resolves issue 4 in nathansobo / treetop
Not sure I understand why it's more complicated than I expected, but you provided a test so I'm assuming it's ok...
I think it might be possible to simplify the logic by allowing a "primary" to be labeled instead of of supporting the special case of a single labeled element sequence, but I worried that modifying primaries to support labels might be disruptive.