Just a follow up...

It could just be a stand-alone program that takes a location and path as input and then generates tiles right into the Mapper database and cache while Mapper is off. That would be the simplest starting point and wouldn't even need any mods to Mapper itself. Sort of like the Map Tile Downloader but, rather than downloading, it just generates them from local vector data.

Hmmm, set mapper to "repository Local" that has a dummy URL. Then, run program to fill said repository with data, at least around a specified location and to a specified zoom. Somewhat of a kludge, but a good first step. I wonder what the Navit code-base looks like...

Still thinking out loud...