I'm happy to see this thing is still in the release plan, but I really have to disagree with the dessign decission to use a GBA cart as storage.
In my opinion, the BEST (CHEAPER, FASTER, most FLEXIBLE and most DURABLE) solution would be a hybrid dessign with RAM and a memory card slot (either SD or CF would be fine).
RAM is cheaper, faster and infinitelly more durable than Flash, which has a significative rewritability limit. If that flash memory is inside the cart, or, like in this case, inside of another different but equally rare and proprietary cart, you have to replace it and that probably won't be possible forever. On the other hand, if that flash memory is in a STANDARD FORM FACTOR, like SD or CF that will be available at retail for a foreseable future, then it's not a big deal. You buy another and that's it.
Also, having RAM in the cart would eliminate all technical problems found in devices that use the a CF or SD card directly, like slowdowns caused by the flashcard's speed being lower than the original cart's mask ROM, other slowdowns and compatibility problems caused by the need to use a loader that is copied into RAM and is an straneous element to the game (designed to be the ONLY thing that controls the console).
With such a hybrid RAM+flashcard dessign, you would have a menu loaded at power-on, and you would select a ROM image stored in the flashcard's filesystem, which would be copied into RAM and run from there, without any other interefering and bug inducing straneous elements.
Additionally, such a setup would enable to have more than one rom withouth having to reflash a cart each time, built in save file management, and many other possible functions.
A simple onboard loading program coupled with an upgradeable system menu in the flash card could bring any kind of possiblilities.
I guess that you have this almost finished by now, but, please, consider this.
EDIT: Additionally, having separated RAM for ROM loading, and the card's filesystem as a means of storage, could be used not only for the menu to load ROM images from it into RAM, but also for RAM resident programs to have a filesystem reader and access the card. This would be really great for homebrew development.
