You can use rechargeable NiCD or NiMH batteries in a Mag without problems, though you may encounter some contact problems using AA batteries in a D adapter. If you do, you'll need to compress the end of the spring a bit to make contact. Mag has had that "don't use rechargeables" on all of their incandescent lights for years and just carried it over to the MagLED lights.
You might, however, want to do a reality check on the use of rechargeables over the cost of alkalines. A pair of D sized alkalines will last for quite a while. Since you're using AA batteries, the cost of the batteries isn't much, but it will be a significant reduction in run-time if that is a factor for you. I do not see much advantage to using D sized NiMH batteries in a light like a MagLED though as it would take at least 8 sets of alkalines to break even, and many people may not use that many sets of batteries in the entire time that they used that light.