[plt-scheme] Transforming expanded syntax from the top

I'm working on an algebraic stepper for the ProfessorJ languages.  My
initial attempt annotates the module produced by ProfessorJ before
it's expanded (via a #%module-begin macro), because this
transformation needs to distinguish "user" expressions from those
composing ProfessorJ's run-time system, and this is more complicated
after expansion.

Transforming pre-expansion, however, turns out to have some problems,
and so I'd instead prefer to transform the module body after it's
expanded (working with the entire body provides the context to
identify user expressions), but I'm unsure how to tie into the macro
system at this point in expansion -- i.e., at the body of the expanded
module.

Any suggestions?

-Casey