I found an old aluminum saucer sled that would make a perfect curved shield, does the ban on metal weapon cores apply to shields as well?

No, it's not explicitly illegal, but be aware that it may be significantly more difficult to make safe. I'd recommend having someone with a lot of foamsmithing & weapon checking experience help out or at least advise & vet your ideas so you don't end up wasting a bunch of time and materials.

I agree with Derian. It's legal, but expect it to be rigorously checked. I don't know the thickness, but aluminum is a notoriously weak metal. Should it taco, or in this case fold, during combat then there's probably a significant increase in chance of injury.

I fought with a metal core shield for years. It is perfectly legal to have a metal core shield. It's really no different than any other cored shield made of wood or plastic. Safety wise it was safer than any plastic or coreless shield by far and and ended up being a little heavier than 5/8 sheet plywood cores. My shield weighed in at 35lbs when finished and I used it for 5 or so years before making a new one. And it only failed once which was fixed by replacing the edge foam.

For a durability stand point for metal shields look at 16g at least to start with. Plus side to aluminum is it can bend and pop out like plastic if it's good quality (I've personally seen one do this.)

So yes make your shield with the metal core and build it like you would a ply wood core. Done that way you should be able to look at the weapons checkers and chuckle as they look for any thing they can find to fail it, because it guarantee it won'be because of the metal core.