Power switch

There's a relatively simple way to add a switch:

Using a fine-tipped screwdriver, pry out the little gold plug for the power (positive) lead of 3 of the ESCs servo plugs. This will disconnect the power supply from the integrated BEC in each of these ESCs. There's a little video on how to do the above process here: http://www.youtube.com/watch?v=clNvfjhMQ5w

Put an inline switch on the 4th ESC's servo lead and voila! Make sure you tie back and insulate the exposed power plugs from the disconnected ESCs so they can't short out.

This process is completely reversible - you can simply push the little contact back into the main plug and it will work perfectly again.

The control board and Rx shouldn't have any trouble at all running off a single ESC's integrated BEC - after all, it's designed to run multiple servos which have much larger current draws.

A slight variation on this is to remove ALL of the ESC power leads, and use a stand-alone UBEC to power the board and Rx. Put a switch on that line and you will achieve the same thing.

However this might turn off the RX, the batteries will still slowly discharge into the ESC over a period if time, I had a large coaxial heli that had a similar switch the batteries were always discharges after a long period to the point of very low cell voltages. This could have just been because of the type of ESC in this heli, but I would check with an ammeter to see if there is a current discharge with the switch turned off, it maybe very small but will discharge the batteries over time.