There have been a couple questions like this recently.
For example someone noticed that the current RB750 has a place for it on the PCB but it’s unpopulated: http://forum.mikrotik.com/t/its-possible-add-an-usb-port-to-rb750/54344/1
The RB750GL also has the same unpopulated USB pads on it’s PCB, see the picture here: http://routerboard.com/pdf/355/rb750gl-ug.pdf
It looks like the hardware design has been done and all Mikrotik would have to do is populate the connector, usb driver chip, and miscellaneous components.