Start air layers at the beginning of the growing season. That is the time it works best, and once roots have formed you still have time to get them established beefore winter arrives.
That being sad.. I am not sure you should put the effort in this plant. The cultivar is not too well suited for bonsai, with long internodes, brittle wood and in general a coarse growth form. I would look for a more traditional cultivar, ideally the 'wild' japanese maple.