So yes and no. The LEDs that u replace your incandescent 120 v bulbs rectify to 12 v. The old school track lighting such a lightoleer etc have a transformer that drops to 12 v but I believe that is also ac. But now if you go to the landscape garden section of ace or home Depot there have recently arrived what appears to be 12 v ac rectified to 12 DC LEDs with the two prongs that fit in all the above applications. But as to what they look like in a headlight I would say try and report back.
Yes if the above bulbs work and have a.decent path it could be good. The thingie with led lights other than the gigantic emmiters on work lights , I think an array is necessary to give great illumination.