This is not a parser problem but a missing feature in the reference resolver. Speaking of parser, there's room already in the syntax for the `: {!t."`Foo"}.
This might be useful to disambiguate between a regular and a polymorphic constructor (as an alternative to a prefix like {!t.poly-Foo}).

We already have code to handle ambiguous references, so it might be possible to handle cases like {!"`Foo"}, which could mean "The constructor `Foo in any type declaration in scope.".