I import a .ucf into an empty PlanAhead project and start to drag and drop I/O ports into dedicated MGT pins. I know these are valid MGT pins, but PlanAhead does not allow me to drag and drop those ports.
PlanAhead does not currently allow for Assignment of MGT Dedicated pins unless the netlist is imported.
To work around this, you must be in "place ports sequentially" mode and go to Tools->Option->General and un-check the option which forces legality checking on I/O placement.
This is only required when you have empty netlists.