The biggest obstacle would be the [] support. Current implementation relies on the readtable feature, which isn't very common among Scheme implementations. You can implement your own read function, which would be much more work than the current bracket.scm.
Other non-standard features include hashtables, threads and some system functions, but you can find them in most full-featuered implementations. (Except break-thread, whose semantics is not in pthread IIRC, so pthread-based thread system may have some difficulties.)
