Hmm.. interesting. I get why you'd want to filter it down, but if you already have support to read in the other format... How hot would it be to be able to actually use that "send to phone" button on Google.

The problem isn't in the tile generation, but rather the overhead in the search for objects in the spacial area. (Said another way, drawing is fast, querying a spacial area on a database with millions of objects isn't.) One optimization I've been working on is a way to pre-feed it a shape file for caching that data for use in subsequent requests. That's handy, since it can then create a sub-set of the data it needs for a given segment and use that for future requests without doing another request. For modRana, you could make a crude shape file for route (say route +/-2k) and use that to speed up the process quite a bit.

Even if it's too slow to do real time, it could do things like filling gaps the existing tile cache for a route. With a little adaptation, it could take the route shape, find existing/missing tiles, and generate just the missing ones. If you have even partial coverage, it may be able to keep up that way at least.

I've worked with two that have it, including the one I'm working on now. In PNG format, you can even make RGBA files, with A being the transparency level. (So you could make maps with 100% background transparency, 40% road transparency, and features (like rail road tracks or labels) at 0% so they stand out.

The hard part is getting/massaging the vector data into a format that it likes. And vector data isn't always small... the global vector map I'm working with right now is near 7G. Nothing compared to tiles, I know.. but...