Update: The Android AOSP has been updated (sometime between 2014 when this question was asked and 2025) to support this according to the docs

When using BLE, an Android device can act as a peripheral device, a central device, or both. Peripheral mode lets devices send advertisement packets. Central mode lets devices scan for advertisements. An Android device acting as both a peripheral and central device can communicate with other BLE peripheral devices while sending advertisements in peripheral mode. Devices supporting Bluetooth 4.1 and earlier can only use BLE in central mode. Older device chipsets may not support BLE peripheral mode.

So according to this, the answer has changed to "yes", Android can now be used as a headset for another device