Thanks for the post!
I've definitely been interested in adding support for Leanpub extensions, but the reason it is not yet included is because no Leanpub Markdown processing library exists that I can interface with. It would likely have to be created first.
Typically an official specification and/or test suite would be created by the creator of the language first, which would allow someone outside of the organization to create an accurate processor (or better yet, if the organization itself released an official library). The last time I spoke with folks at Leanpub, they had intentions for creating an official specification and perhaps a Markdown API (which I could interface with), but with limited resources/funding they were not clear on when this would happen. I haven't followed up with it for some time, so try to do some digging to see where things are at.
Aside from an official Leanpub Markdown library, I'll make a note to see if I can do something in the meantime to get you the features you're looking for.
What are the most important parts of Leanpub Markdown for you? I'm not very familiar with their varietal.