I've been reading up on iGo and Mio over on GPS Passion and it looks like we should be able to add our own TTS voices with just a little work. I spent a couple of hours playing around with it this morning following the instructions in this thread and couldn't get it to recognize the new voices, so it's not the exact same process, but it should be something similar.
Download Additional Voice Files
You can get additional TTS voice files here:
Modified Samantha (Removes Route Recalculation warning and a few other changes)
Voice files are stored on the device in MyFlashDrive/AAPL2/iGo/ScanSoft/RealSpeakSolo/speech/rssolo in zipped format.
I'll post more updates as I uncover more information.