Not possible. To create a ROM, kernel sources are required. Coby never provided source files as they didn't build their tablets but sourced them from a Chinese manufacturer.
In addition, later Generation 3 and all Generation 4 models incorporated an e-fuse that would be tripped if the firmware is modified. At the time Coby went out of business no one had been able to bypass the fuse. Tripping the fuse would render the internal storage non-functional.