You could do that but this is one of the rare applications where alkalines probably make the most sense, unless you're frequently running the light for long periods. With normal usage alkalines last a long time. I think I've only changed mine once a year or so, not counting a couple that I ran to exhaustion on purpose for testing.
Alkalines will probably be dimmer than nimh over a fair amount of the runtime curve but they won't crap out as suddenly. You'll eventually notice that the light is getting dim, but you can replace the cell when you get around to it and the light will stay useful til then. When the nimh dims, you'll have much less warning.
These lights are not about throwing a lot of lumens and it's not really worth trying to maximize the lumen output. There are better ways to do that, if that's what you want.