i don´t mean to be funny or ignorant, but to get a just intonation scale, you´d basically use the [/ 0.] object.
there should be formulas around for mtof and ftom - try to find where they contain a "12." - and see what happens when you replace that with "13." or with "4.", this is another solution to it, as long as you want to be bound to the octaves system. if you prefer scales without multiples of 2. you might hit a border here.
can you say "trans europe express"?
oh, and dont forget to mix just scales, like setting up a /3 per octave and a /8 per octave in parallel, where the sum of both series makes your scale.
@Roman - just intonation has nothing to do with the number of divisions per octave. If you use an equal temperament formula and simply change the '12' to something else, you'll simply get a different number of equal-tempered steps (per octave, or whatever your repeat factor is).
@federico - the math is pretty easy. Just define your 1/1 (starting point) as a given frequency, then apply just intervals to that with [*] and [/]. For example: if you start on A=440 and want a just perfect fifth above it: 440 * (3/2); if you want a perfect fifth two octaves above it: 440 * 4 * (3/2); etc.
You can make your own abstractions pretty easily to do the math for you. Somewhere I've got a 'just' [mtof] abstraction which will allow you to specifiy the 12 ratios (assuming a 12-step octave repeating scale). I'll see if I can dig it up, but it's pretty straightforward.