What about converting graphics to simpler formats, like those in Wesnoth? That would promote modding. Say on the first run OXC could dump all data to PNG, WAV and CSV, and then uses these files. Now modding would be as easy as opening CSV in a spreadsheet app. That way supporting HD-graphics would be a matter of adding support to blitting routine.
Modders already use PNG, and there are very easy to use utilities that convert original resources to GIF/PNG too.
Modders already use WAV, and all original sound effects are already in WAV format too. They are packed in an archive (CAT), but it can also be easily extracted using a tool (not that you would need that for anything really).
Modders already use YAML, and all vanilla rules are already in YAML too, no conversion needed.