The SSAX parser and the SXML transformation tools have been generalized to vanilla R5RS Scheme and then specialized for Scheme48/SCSH and the upcoming PLT Scheme 200. The specialization relied on native module systems of Scheme48 and PLT Scheme (resp. functors and units).
This porting and specialization is the work of Michael Sperber. His message (as of Apr 18, 2002) on the SSAX-SXML mailing list gives more detail.