I actually misunderstood what the Custom Markdown Processor feature was for. What I was looking for was a very similar feature but not one that replaces the built-in MD processing but one where I could place a pre-processor between the file I'm editing and the one that Marked receives to display.

For example. I want to place MD text inside block comments --[[ --]] in a Lua program. When I save the Lua text, I'd like Marked to spot the file has changed and then run it through the supplied pre-processor which (in my case) will remove everything except the contents of the comment blocks; thus piping only MD text through.

I would have thought this might be a useful feature in general?

best regards