Yes you can, it has a low voltage warning at 2.8v. It’s designed to use protected cells so expect battery rattle. A bit of insulation tape around the cell can solve that. If you drop the light while it’s on it might switch off due the lack of spring compression when using the shorter unprotected cells. I did a drop test on a carpet. So I’m sticking to protected cells in this light.
Using XPLs, you will get more output at the expense of runtime and less constant brightness.
Using Nichia 219Cs 4000k 90CRI+, you will get slightly less brightness in exchange for much longer regulated runtime and even better color temperature.
This is a fantastic light. I am really enjoying mine. The only niggle I have is that the clip is not deep carry. Other than that rectifiable qualm, I really like everything else about the light. Buy one, you will not regret it.
Also did some experiments with emitters, optics and filters. Spoiler: you can get the XP-G3 to look good with a frosted Carclo and a minus green filter but at the cost of output. LH351D is still better.
Update 2018-06-11 22:00 UTC: Wuben just set a hard deadline for this GB: Everyone must make their orders / finish their payments in the following 24h. After that, all unclaimed lights will be shipped to me. (No exceptions, no payments after that deadline!)