There might be a light out there with cell protection for NiMH, but I don't know of it. This is mainly because Bad Things can happen when you try to recharge over-discharged lithium cells - it is possible to make NiMH cells go bang, but you do have to work at it. The RC modelling crowd are downright vicious to their batteries and have documented every possible nasty thing that it is possible to make all sorts of cells do - but then they will do things like try to ram 50A into small cells to charge them faster. On the whole, nickel based cells are far safer.
With NiMH the discharge curve is pretty flat till it falls off a cliff so the usual advice is to recharge them when they start to dim. Once the light starts to dim its output will drop like a stone anyway so it will be obvious in all but the lowest power consumption lights that they are close to the end of their charge anyway.
This is a typical NiMH output/voltage chart.
As you can see, it is very obvious when the cell is running out of power.
And here's a typical alkaline discharge graph at a reasonable current.
