BennVenn (https://bennvenn.myshopify.com/) is releasing a GBZ product that will allow you to play a game from an actual cart by making its save file and rom readable from a USB interface like a flash drive.
This could be used in your GBZ project to allow you to play game off of actual cartridges. Not sure how it works or how easy it would be to implement into retropie, but now there is the hardware to support the option
