The developer of Marked, Brett Terpstra, has created a Ruby script which makes it easy to get a relatively real-time preview of the portion of the outline you are currently editing in Scrivener. For those not familiar, Marked is a neat little application that simply just sits there making a Markdown file on your drive look pretty, and updating itself whenever the file is saved. In essence, it can give any editor on your computer a good preview window. With programs that autosave frequently like Scrivener, that can mean only a little lag between what you write and getting a crisp MultiMarkdown rendered display of it.
Details can be found on Brett’s blog.
The RTF files that Scrivener uses are monitored for changes, and the most recently changed RTF file will be processed with textutil to strip out all coding and convert it to plain-text. This means there are a few practical limitations depending on how much of Scrivener’s MMD generating features you use. Footnotes and images will not display, for instance, since they require the compiler to generate these them fully. Additionally this will not be useful to you if you use rich text formatting to set apart your paragraphs, with the intention of using the compiler’s format-to-whitespace conversion utility.