Is there any way to see the battery level of a Bluetooth device (e.g. magic mouse) in Windows with Boot Camp?
There is an application for Windows called "Bluetooth Battery Monitor." It is in beta right now, but I use it for my Ghostek SoDrop 2.0 headphones, and it works well.
You can download the beta at: https://www.bluetoothgoodies.com/
I don't think that BootCamp per se would make a difference as to whether the Windows program would work or not, or any program at all, for that matter.
Our software Magic Utilities can read the battery state of all Apple Bluetooth devices.
Besides this, we offer much better multi-touch and keyboard support than BootCamp and run on any Windows computer.