APN Hoedown is a Markdown processing library written in portable C.
A critcl generated wrapper for hoedown is around is available from my fork of the project on github. To build, checkout the vc6-compatibility branch (the wrapper has not been merged into the master branch as yet though it will build on any critcl target platform).
Documentation is at https://github.com/apnadkarni/hoedown/blob/vc6-compatibility/tcl/readme.md .