Omni does provide XSL templates for transformation to HTML.

That is how the HTML export works. If you did want to find the XSL files, you could take a deep breath, exercise caution, ctrl-click the OO application file, choose Show Package Contents, and drill down through the Plugins sub-folder inside the package. I don't really advise this, incidentally.

Or, more sanely, you could develop a work flow that involved exporting to something like HTML, RTF, or PDF (arguably the best for Devonthink), perhaps automating some of the steps with Applescript.