I'm the author of the Clojure->Scheme compiler. I'm looking into the possibility making the compiler not require the JVM for macro-expansion.
Clojure makes use of vectors and maps directly as syntax -- I was looking into using Gambit's bracket-keyword to produce literal vectors from the reader, but it seems the reader just attaches a symbol to the head of a list, as opposed to evaluating the forms.
I'd like the macro-expansion stage to have access to the actual maps, vectors etc. as some Clojure macros behave differently depending on whether the forms are lists or vectors.