Ive been looking for exactly this as well. useful when you need to set an extra ssid like, r720-5g AND r600-5g (for testing speeds or BW of a specific AP and/or specific band). This is possible ofcouse with standalone, ZD, and vSZ- so would be nice on Unleashed.
(im thinking one could do this via unleashed CLI in AP-mode, but im not sure, and i dont know if it would last trhough reboots, if even possible)