Ow. Would it be too much to ask that it accept any sequence of opening brackets

Not to much to ask at all. At least, not as long as you can be satisfied with multiple repetitions of the same delimiter character. From the latest update of Synopsis 2:

• For all quoting constructs that use user-selected brackets, multiple, adjacent, identical opening brackets must always be matched by an equal number of adjacent closing brackets. Bracket counting naturally applies only to sets of brackets of the same length:

      say #{{
          Comment contains unmatched } and { { { { plus a counted {{ ... }} pair.
      }} q<< <<woot>> >>   # says "<<woot>>"