The new 5.1.0 eink drivers add a new eink update mode. It is 1bbp (pure black and pure white) and they named it mode AU, which in the comments calls it "ANIMATION UPDATE MODE". Hmm... I wonder if they got that idea from the mobileread forums...
EDIT: I found a reference to "animation update mode" in eink_panel.c for 5.0.0 -- apparently they added it to more source modules now, so perhaps they plan to use it some time in the future. It is still missing from mxcfb.h though...