I've wondered about this too, but CantoDict includes rough/pockmarked/spotty in its list of meanings for 麻, and it does seem at least plausible that this could be related to the roughness of hemp cloth. The meanings it gives for 痲 appear to be a degree more "serious", i.e. leprosy/measles rather than just pockmarked, anaesthetised/paralysed rather than just numb. I've never seen 痲 on a menu (this of course doesn't mean that it has never appeared on a menu, particularly since a menu using simplified characters would use 麻 anyway).