OK I understand the extent of the problem. Unless someone decipher the new Garmin format there will be no quick solution in sight ... It might be unsolvable if it's encrypted.
I did note that rebuilding a map inside MapSource does generate a .gma file. Coupling this with an .unl file from JetMouse KG might work (but not in newer GPS) ? I don't know if basecamp does generate as well a .gma file ...