Actually, “A”s and “M”s is the accepted standard for dealing with such things. “A”s and “M”s aren’t words in the strictest sense, they’re letters, and when dealing with non-words, it is accepted practice to put them in quotation marks, with the plural “S” on the outside, since letters don’t have a plural form in written English. Also, “As” and “Ms” are already taken as a word and an abbreviation, respectively.