Cool idea, although it doesn't really change anything as far as the user is concerned. The time saved passing one base10 number instead of nineteen base4 numbers might be small in comparison to the time required to hash those nineteen base numbers.
Still, I wonder how much information you could store in a hash like that. It seems like the lua interpreter in toribash is compiled with standard floating point doubles for numbers, which means it can accurately represent ints up to 10^15. With 19 base 4 values the biggest your hash can get is in the 10^11 range, which is getting up there.
Also, you could just make the thing hash 20 values instead of 19; the last base4 variable can store the grip state for the hands: either (off, off), (off, on), (on, off) or (on, on), corresponding to 0,1,2, or 3.