There's no way you would want to rely on the winch if it were only secured to the alloy bar.
However, you can brace up behind the alloy bar and bolt the winch to that. 6mm steel plate is the recommended minimum support for a winch.
Don't forget that the winch will not only pull the weight of your vehicle, but it will pull that weight out of whatever you've gotten stuck in, so the strain is quite considerable.
Alloy on its own just doesn't cut the mustard. I'm sure I've seen someone with an alloy bar and a winch - they had built the framework up themselves, if I recall correctly. Not sure if it's something available in a store.