I don't look forward to the day when I have to search for something beyond 256 GB. As it is now, I'm about 10 GB shy of max'ing out the 128 GB ZIF SSD I have in my Rockbox'd mp3 player. There is a 256 GB ZIF SSD available, at least for now.
For what it's worth, some of the reviews for the mSATA-to-ZIF adapter I linked to are from iPod users who used the adapter to upgrade their storage. I imagine they might be running the official firmware, though, and not Rockbox.