Perhaps you could style the editor and text as whatever your preferred version of plain-text is (a nice monospace font, for example?), and then compile the document to .txt.
For the time being, I’ve set up my Scriv using Options > Formatting with Droid Sans Mono 12 (click in somewhere in the text entry example with the Nietzsche quote and then the A button in the top-left of that section to set the font, size, and style). A well-scaled monospace font makes MultiMarkdown composition easy and crisp. Click “Use Formatting in Current Editor” to apply changes to the current project. This will cause new projects and documents to be created with this ‘plain view’ be default. (You can also use the “Manage…” dropdown to save your theme adjustments for later use, in case you want to have a variety of theme presets.)
From your newly ‘fake’ plain-text editor, copy a chunk of text and use it to define a formatting preset (Format > Formatting > New Preset From Selection). Name it something evocative, like “Vanilla Text” or “Markdown Style” or whatever. Now select the Draft in the Binder, switch into Scrivenings mode (View > Scrivenings), select all your text (Cmd-A or Edit > Select All), and apply your newly-created Vanilla Text preset (Format > Formatting > Apply Preset > Whatever-you-named-it). Voilà! Plain text faked.
For the actual generation of plain text, once your composition is complete, you could Compile the document to Plain Text (thereby retaining any comments, inline footnotes, etc. in your Scriv doc, but stripping them from the compiled file). Or if you don’t use those features, just select-all and copy-paste into TextWrangler or whatever your favorite plain-ol-vanilla text editor is.
So, I suggest that Scrivener already has the ability to fake plain text, without too much work-- and you can get it to look exactly how you want.
Finally, there’s the option to compile to Markdown, and I’ve heard (but haven’t tested it myself) that Marked2 can preview live Scrivener files to show how the Markdown is coming along. These might be additional features to check out if that’s your kind of thing. There are quite a few people out there with blog entries on using Scrivener to manage their Markdown blog posts, you might check those resources out, too.