The internal flash is a MMC-on-a-chip, or an MMC without the housing. Check the datasheet section for details:
http://www.rockbox.org/twiki/pub/Main/DataSheets/SanDisk_embedded_flash.pdfNote that is has a BGA package, very nasty for DIY.
The one which is in there is the largest model from Sandisk. I guess this "TriFlash" format wasn't very successful.
You could replace it with a real MMC card. The Ondio has "plenty" of space between the tuner board and the main board, an MMC would fit, I've checked. The only problem is that a real MMC has no reset pin, something we need for switching modes. You'd need to make a little circuit that cuts its power instead. This is slower, the software needs adjustments.
Jörg