Wait, doesn't this already work? Not for the controllers (open xr spec issue) but the headset battery should show up fine in SteamVR and any SteamVR overlays (xso, ovrtk, etc). I use xso in vr and it shows the percentage on my wrist and warns me with a notification when it's low. I suppose it doesn't work outside of VR though, if thats what you're talking about.

The Passthrough environment on VD eats up my battery too fast and I like to keep an eye on it But it is a hassle to press the trigger button, move the controller laser to the menu button, click it, see what my battery is, move the controller laser again, press the menu button to close it, and then press the trigger button to hide the desktop toolbar again. It would be great if the first trigger button press could display my battery percentage on the desktop toolbar, or if like the OP said maybe have a way to display battery percent on the PC's system tray. Either way, it would be very helpful and waste a lot less time and make it easier to monitor the battery situation.