Wait, are you talking about a bull float or a hand float? Because a magnesium-aluminum mix is pretty common for a bull float, but I've never seen a hand float made from that. Magnesium hand floats are standard for a reason, they don't drag as much on wet concrete. An aluminum hand float would stick like crazy.
You've never seen a magnesium-aluminum hand float? That's wild... they're all over the place here. Every supply house carries a few models, right next to the pure magnesium ones. Sure, the pure stuff glides a bit easier, but the mix is cheaper and still works fine for a lot of guys. Maybe it's a regional thing, but they're definitely out there.