Markdown inline syntax is straightforward to capture using SGML SHORTREF. What's more difficult (impossible) are things such as reference links where a markdown processor is supposed to pull text (the title of a link) from wherever it's defined before or after its usage.

Haven't heard about archforms in a while ;) but it's not a technique for custom syntax, and since markdown is specified as a Wiki syntax with canonical mapping to HTML, there's no need for the kind of simplistic element and token renaming possible with archforms.