I don't see why it wouldn't work. It definitely wouldn't damage it since it's a lower overall voltage (4.2V vs 6V). The question is whether or not you'd acheive the full 320 lumens from it or if its output would be slightly reduced (I can't imaging a significant drop). Runtime should roughly be the same based on each battery's watt-hours, that's assuming there's no difference in output. Do you already have a charger. One thought would be to get a 4.35V Li-ion 16650 battery (link below) for the little extra voltage but you'd have to have a charger capable of charging something at that voltage. But there are some cheap single bay options if you'd want to go that route.
If I recall correctly, Maglite told me the voltage of 2 RCR123s/16340s is too high for the driver; and it was designed for CR123s only. I had a single 16650 I tried in mine, but it only came on w/ one super low mode, maybe a fraction of a lumen. For that reason, I sold it.
