Did you have any luck with this? There might be a BIOS setting in the menu that allows you to disable the SMM lock, and as icon suggested on IRC the speed racer exploit might also be a feasible workaround.

Another idea might be to try using coreboot on this board and simply not set the SMM lock bit until you need to do so ;-) There's a decent chance the Bayley Bay CRB implementation will work, though you may need to do some customization for your board: https://review.coreboot.org/cgit/coreboot.git/tree/src/mainboard/intel/bayleybay_fsp