This is possible on Android, at least on Lollipop. The feature is dependent on the Bluetooth headset sending the battery level in the right format.
If the headset supports it, you will see an option in the settings screen for the Bluetooth device. The option will be right above the check-boxes for Call and Media audio. If you don't see this option, then the headset either doesn't send battery data or it doesn't send it in the correct format.
Once the feature is enabled, you will see a fill-able icon on the left side of your notification bar. I've confirmed this using a cheap earbud set from China.