From your link there looks to be two functions spare, I guess you would need a new PROM to make use of them though. The MIT Lisp Machine does just map microcod

From your link there looks to be two functions spare, I guess you would need a new PROM to make use of them though. The MIT Lisp Machine does just map microcode instruction bits onto the 74181 function pins but it has much wider microinstructions than the 32-bit Alto ones.