Dunno about the FMGS, the FMC certainly does in a boeing. We had an incident where some very curious captain (PNF) surprised his FO during cruise in a 738 by entering an MAC of 36% into the FMC when it was in really around 20%. It resulted in overlapping yellow bands and immediate stickshaker activation, flown stall recovery and lateron some very embarassed explanation at a tea-appointment without biscuits.