I used both straight and flex coupling on the driveshaft of the mower U-joints. I find that when the deck is removed, and i move the coupling 90 degrees the zerk is right there point out and ready for greasing.
I suspect you are trying to do it while its still attached to the BX. IF you don't want to remove the deck, I suggest disconnecting just the MMM shaft and slide top half off and grease it on the ground. before reinserting- you should be able to turn the lower half of shaft so lower u-joint zerk pointing in a way you can grease that one too. Its doable.