James

Set Display->Battery Meter to On All Screens to display it on top of your custom screen.

I should add that while the Windows serial driver is proprietary, the protocol it speaks is standard USB CDC serial. So Linux will recognize it without a driver. Because the device is composite, the CDC serial must have an Interface Association Descriptor, which makes the serial not work with Mac OS X (no IAD support). But, the HID-tunneled serial will work on every platform.