As far as I can tell MaemoMapper simply reads out directions of a route point when you get close to it. If you can get istructions with finer detail I'm sure MM will happily use festalon or such to read out the instructions for you. :-)

The routes I've tried come in XML and as such are easily editable. If you want to experiment with MM and routes that's an easy way to start. In my opinnion however it would be better to forget speech synthesis and find a way to receive coded directions. What I mean is that it would be better to receive driving instructions in machine readable format instead of plain English. That way it would be much easier to use pre-recorded voice samples (e.g. borrow the samples from Maps). As you probably know, consumer-grade speech synthesis isn't very good.