The warning led is definitely in parallel with the gun motor, and in fact it light up when the motor is pulling back the airsoft piston. It might get you wrong the fact that they might share the same wire from the connection point on. But they are in parallel. Therefore the warning led is under 7.4 V, and that is the reason why, if you check , you will find a small resistor in series with the led, to drop the tension.
You could replace the led with something to charge any glowing BB, but if I can give you some advices, build a separate circuit to charge the BBs, using a button-type battery and a UV led (they exist) as I did in both my Pershing and my 1/18 Panther.
If you try and use the warning led connection, since you cannot put the led inside the piston or the barrel, you would have about 1 second to charge the BB before it is pushed into the barrel by the forward piston, provided you put the UV led just over the end of the loading ramp. Too short a time to properly charge the BBs.
Instead, if you put a switch-operated UV led in the loading ramp (I used the safety switch), you can charge the BBs for several seconds, or minutes, before shooting them.