Other than demonstrating that it is possible, there really is no reason to have any motors involved. If a higher voltage than that of the drive battery is required, there are step-up circuits available. If a 5 volt supply is needed, which is fairly standard for anything USB, the ESCs BEC does that job perfectly, in fact, a stand alone uBEC would probably be better as it will be able to supply more current without wasting as much energy. For other voltages, a wide range of voltage regulator ICs are available which give adjustable voltage. Motor and generator inefficiencies will waste half the available power. This is the main reason why rotary converters fell out of use many years ago.
The ESC in the link claims to be able to use 6-10 NiMH cells, 10 NiMH is nearly enough equivalent to a 12 volt Lead Acid battery. Used in mode 1 it will not know the difference - 12 volts is 12 volts wherever it comes from. It will need a control input before it can give an output. This can be supplied by using a servo tester, but the tester must be very carefully calibrated since it is easy to give a signal that is outside the range that the ESC programming expects, and results would be uncertain.