In Racket it's "#hash" followed by an alist, so we are back to the idea of alists.  Clojure uses the same syntax I threw together, except starting with "{" instead of "#{".  Of the two, I prefer Clojure's.  How do you feel about it?

As for bytevectors, I'll add them using R7RS syntax.  Should we require hex?  It's easier to read, if wasteful.  Chibi outputs every value in hex except 0, which is exceedingly common.