Looking up Marwick's etymologies there are several of this unexplained placename element, including two Mount M(a)iserys and three Maeshowe cognates [2 of How(a)mae and a Maizer], but did not elucidate matters. Then on a different matter I researched the Proceedings of the Orkney Antiquarian Society and came across his Sanday article. He noted that the mae was very frequent in Sanday. This island is famously long and low, so I think definitely maes=plain as suggested. Spoils the mystery, alas.