Eh, I still don't like that diagram, at least I would test it first beofre I put it with the rest of the electronics. What I don't see is a resistor in line with the fan. The fan and diodes should have some small resistance, but really it looks like you will just short out the fan/batts again. Maybe fans don't need R's? Not sure, but I wouldn't bet my esc on it.
What I would do it just tap a line right off the ESC where the input voltages go in (solder on top of the batt wire connection) and run it to the fan with a resistor (and diodes if needed.)
Here is a 12V 40mm fan It needs 120ma. So for 16.8V: (R=V/I) ~16.8V/0.12A=140 ohm. (150 ohm closest iirc.) If you use diodes to drop voltage you will need a lower R.
Note also that P=I^2*R. P=(.12^2)(150)=2.16W. You need at least that high-rated a resistor, or at least a combo of smaller resistors to do it. Ratshack has mini-resistors upto .5W, then bigger sandbar resistors.
If you do it this way, you don't need a switch to the fan, or long wires going everywhere. It will only turn on when esc is plugged in. You may switch your RX, but don't leave batts plugged in anyway.
Useful link with calcs
I would first do this with just the batts and fan. If you don't blow anything up, then integrate w/ $$$ escs. Or just get a 5v fan and a y-cable and tap off the rx and leave it easy...
G/L