Just a thought, how many USB ports would be available in addition to the external charging port? If there were the prototyping module option under a Mugen cover we would want to have hub logic and mode switching for every USB/charging interface pad so we could for example charge via the port and control a plugin SDR module.
As for Mugen covers I don't think they are easily available anymore without a expired $80 battery. My 3d modeling skills might be able to design a printable replacement but it would be cubist at best no curved lines.