It's not quite what you're looking for, but I think the whereigo game on Garmin can be scripted (Lua).
Unfortunately the thing hasn't taken off anywhere, there is a ridiculous licensing agreement to generate an output file for it using the maker's tools, and by running the whereigo engine you seem to lose all the normal mapping features.
But technically you could script it specifically for something in that (but you'd lose the normal mapping functionality)
Meh, I agree I'd buy it.
But from their point of view it's development effort, they don't know how much it will sell extra, and it could be a support nightmare. "My garmin battery doesn't run for as long as you say it will, or doesn't navigate right - yes I have a random script off the web running, what's that got to do with it??"