Did you consider pre-rendering stuff client-side using SVG? I assume adjusting a few parameters in a JSON file is easier than going to a site, pushing buttons and downloading and replacing files.
We're looking into every possible idea we can to shave file size and images down for mobile. Look for very aggressive use of CSS3 that falls back gracefully. We're hoping to eliminate all images (!) or radically reduce their use compared to current ThemeRoller. Using SVG is an option on the table for some elements but I think we'll end up using a TR style tool for this to generate static CSS for performance and compatibility.