Interesting.
Could yo incorporate a small tactile switch into the design that would totally turn off the device when it was in the tool holder?
Somebody makes a very nice supercap (Linear tech
https://www.electro-tech-online.com...urnal-V21N4-03-df-LTC4425-GeorgeBarbehenn.pdf) charging circuit that uses two super caps. Could you use PC board probes (look up pogo probes) to connect with the gizmo while it's in the tool holder. What can then happen is the tool would charge the caps while in the base.
Here is a possible spring loaded spring probe:
https://www.alliedelec.com/search/productdetail.aspx?SKU=70009131
Ideally, before using a battery powered tool, you would need a battery check button. Another option is a rechargeable LiPo battery.
If the orientation of the sensor and the detector on the quill would stay the same, then the IR approach should work.
In the case of the LiPo battery, you could keep the IR LED on all of the time. For the cap case, the on when off makes sense.
To add a little wierdness (more complicated), you could use a processor such as an ARM or PICAXE. Some of the ARM processors can basically shut down totally, but allow certain input pins to be used for an interrupt. I'm not sure where the idea fits, but sometimes it's worth throwing it out.
Bear with me. These ideas are just stubs for the basis of a design.
As for you slip-ring idea. I used pogo type probes as sliding contacts on a PC board tracks for a model gantry crane that I did for somebody.
An interesting problem.
I'm kinda picturing an IR remote control being carried by the tool. Not exactly, but you get the idea.