You could get something like this with lua and openresty.
Looks like there are already some lua modules for markdown to HTML [0] and static site generation [1]. It could definitely be done.
But what would be the advantage of rendering markdown to HTML on the fly? It's unlikely to change once it's been deployed.
[0] https://github.com/bakpakin/luamd
[1] https://luapress.org/