Your theory about creating a new ROM from a close relative makes sense.
However, the Tab-720 and Tab-730 are not close as they have different processors. The Tab-720 is based on the Infotmic processor. The Tab-730 is an All Winner.
That's entirely correct. You would have to start by tearing apart the stock rom and then reverse engineer the bootup process, then port a recovery, then you could load a compatible rom build with patches for your device hardware and firmware.