Following a conversation with Tantek and Kevin Marks on IRC, I have patched the OPML Editor (and, by extension, Frontier) to produce XHTML with embedded XOXO (the outline microformat) using the "View in Browser" function in the File menu.
To install xoxofrontier, you can get the code here. It contains three files - a README, and then two 'fstc' formatted patch files - xoxo-1.fstc and xoxo-2.fstc. Open them in that order and it should patch the code in your OPML Editor. You may want to just restart OPML after doing so in case they don't compile or something. It's licensed under the GPL (whichever version is used by the existing code), so if you are compiling a distribution of Frontier, feel free to include it.
For future reference, the code that is changed is: system.verbs.builtins.Frontier.tools.data.windowTypes.outlinerFile.saveAsHtml and system.verbs.builtins.html.getOutlineHtml
I have marked up my changes using comments and clock.timestamp().
All the usual stuff - no guarantees that it'll work, use at your own risk. Have fun XOXOing.
