External BT microphone is a software issue
Since bluetooth hardware is available and working for other devices, only software stands in the way of having full support for bluetooth headsets (includes microphones and stereo audio output). Zebulon is correct, the analog audio (headphone) plug apparently does not have support for mic input, and this limitation cannot be resolved without changing the hardware. However, bluetooth has no such hardware limitation. Also, USB has no such limitation, but USB headsets may be inconvenient due to the required wiring.
I have heard that USB headsets may work with gtabs, but have no personal experience with this.