I checked with one of the engineers, and most of the HTML export is done programmatically; you could do some limited customization, but only by altering files in the app package itself. There are some rather big downsides to that approach, obviously. :-)
I'd urge you to send the
support ninjas an email letting them know what features you'd like to see a template system support. That way, they can take your suggestions into account while planning their work on future versions of the app.