It occurs to me that I can actually type markdown text directly into the MarsEdit editing window and then convert that text directly (in-window as it were) to publishable html via an OS X service called:
md - Convert - Multimarkdown to HTML
This is no doubt the absolute fastest way to use markdown in MarsEdit
There’s a whole bunch of similar services available for the Mac and they can be had from this site. Brett Terpstra has provided a truly valuable service in publishing this material.