Offhand, I would expect M-O to be OK, unless you are dealing with very severe loads, for which M-O "blows up." Anything else you might try would be expen$ive, like detailed numerical analyis. We've run into this issue of late with spillway chute walls that act as retaining walls for a dam embankment. For those, we have sometimes gotten into coupled analysis of the concrete walls and the fill.
Also, M-O is conservative for high walls whose natural period is much longer than the typical period of the ground motion.