Simplest circuit to light an LED in parallel or series to a Li-ion battery being charged
I will just post this comment from above as an answer as handyguy confirmed that it could be useful. Still, somehow drawing out a wire wth the control signal will be simpler of course.

Things to pay attention to:

The color of LEDs D2-D5 has to be &quot;redder&quot; than that of D1. It wont work if those are e.g. blue LEDs and D1 is green. A safe bet would be photodetector diodes or red LEDs.. The area is not so important because the necessary current is small. Still bring them as close as possible to D1. There are 4 Diodes drawn because the total voltage has to be enough to turn M1 on.
The transistor M1 is an N Mosfet. Choose the smallest type you can find with the lowest gate charge possible. R2 is to turn off the Mosfet eventually, but might not be needed if the leakage current turns it off fast enough. If the MOSFET has enough ON-resistance (at least a few 100 Ohm), you could even skip R1.
Shield the assembly from external light, or the transistor will never turn off.

EDIT:
As mentioned in the comments, it is also possible with fewer parts:

Here, D2 is a single photodiode or LED in photoconductive mode. R3 has to be tuned a bit to work. It has to be small enough that it does not turn on the MOSFET due to D2's dark current. And it has to be big enough, that it does turn on the MOSFET with light on.