I don’t think a smart tailcap is possible for the FW3A without a lot of effort. You need a good constant power source to measure.
The main electric path in the tailcap (from what I can tell, that’s the negative end of the battery to the spring to the rim of the PCB to the main body tube) in uninterrupted. That means that the path of least resistance will always be that direct path, so you can’t put a smart tailcap in parallel - no current will flow through it.
The second electric path is possible where the e-switch connects. I imagine the e-switch is connected to a pin of the MCU that is set to “input pull-up” with means it’ll be positive with a ~30k Ohm internal resistor connected. Pushing the e-switch connects it to ground momentarily, which the MCU recognizes. I have before (in the D25 headlamp) connected an LED in parallel to the e-switch. That allows for a “dumb” illuminated e-switch. It didn’t work with green LEDs, but worked with blue. I’m thinking because of vF differences. I’d have to do some testing to see if that’s a viable option for an illuminated tailcap.
Doing a “smart” aux board under the TIR (like what Lexel has done) would be much more feasible and predictable.
Edit: Scallywag beat me to the punch, although I included a few different details. Thanks for chiming in (and confirming my initial thoughts).
My first FW3A-series light should be any day now (I think it just passed through customs) so I can poke around more later, but I don’t expect any surprises.