I just got this same idea as jray3 had here. I have some LiFePO4 cells kicking around, so why not assemble them into a 12V pack (say 6p4s), put a 3000W inverter on them and then order a 2500W Elcon from evassemble.com, which would charge the main pack while on the move. I'd tap into the inverter input, as siai47 has figured out.
Actually having a 12V booster pack opens up a couple of possibilities. In addition to charging it from mains with 15VDC power supply, one could easily charge it on solar or, no I'm not nuts, the DC/DC converter in the car.
Ok, now that you think I am crazy, let me explain. You could use the DC/DC converter in the car to charge the 12V booster pack while you were charging the car, either slow or fast. The DC/DC converter is always running whe you charge. So even at a CHAdeMO station, in 30 minutes, you could maybe put 500Wh into the booster pack, if I recall correctly that the DC/DC converter puts out about 80A. It wouldn't even slow the charge down, because CHAdeMO slows down very soon on i-MiEVs as the max voltage is reached. While charging slow you could of course get the whole booster pack charged while you charge the main pack as well. Obviously you'd have the DC/AC inverter disconnected while doing this.
It would be much more efficient to skip the DC/AC inverter and run the charger directly of the booster pack. I have a Elcon 2500W set for a lower charging voltage, so I think I might make a higher voltage pack of my cells and see if the Elcon would run off of 70-90 volts DC directly. Then I'd of course need to have another Elcon to charge the booster pack.
Edit: Small problem I realized with running an Elcon directly of the booster pack. If the input wires are 1,5mm2 then at most I could draw 810 watts off a 90 VDC pack ((1,5mm2*6A)*90V=810W). That's assuming it works in the first place. I think I'll need to ditch this idea and just go with the 12V plan.