I'm definitely interested in adding support for additional Markdown processors, and I plan to make more available whenever possible.
Specifically regarding Leanpub, I contacted the founders a few months ago to talk about their proprietary implementation of Markdown, and to identify how I could add support for it in MarkdownPad.
Unfortunately, the Leanpub team is just starting the work to formally define their spec, meaning that a real-world implementation of a processing library is still a ways out. Once they formally define their Markdown spec, work could begin to build a processing API (either local or web-based) to perform the conversion.
I can't say for sure whether Leanpub themselves will eventually create a public API for their Markdown implementation; due to their limited resources, it may come down to some enthusiastic community members building some type of open-source implementation based on their spec. Regardless of what happens, I'll keep my ear to the ground on the issue and do whatever I can to help add Leanpub support to MarkdownPad.
Regarding other Markdown implementations like MultiMarkdown: Same story - the only real limiting factor right now is lack of access to an API. It would technically be possible to shell out to MultiMarkdown, wait for its response, then post-process the converted data, but this would all be done outside the context of MarkdownPad. An ideal solution would be if a .NET API was made available for MultiMarkdown. I'll continue working on the possibility of adding MMD support through other avenues in the meantime.
Thanks for the feedback!