Yeah, your house footings even if it was a fire or what not probably aren't going anywhere any time soon, so cementing in concrete is a decent idea. Plus, no one's going to be digging that up while you're there, unless someone pulls some Prison Break esque plan on you, but that's incredibly unlikely.
All they need is metal detector to see if something is hidden inside walls or floor, so I think this could be main negative side of using any metal based seed words backup.
I am not saying someone would scan for seed words with metal detectors, but they could used them to scan for other precious metals, so this this would be biproduct.
Drifting way OT, but in a lot of new construction you see metal mending / binding plates where 2 pieces of lumber meet:
https://www.homedepot.com/b/Building-Materials-Building-Hardware-Mending-Plates/Prong-Plate/N-5yc1vZasc4Z1z1at5cNail them together and then use these for more support so metal that can be found with a metal detector in walls is not a big deal in a lot of locations.
With that being said I think in most locations a piece of metal like this is going to be unnoticed.
Which is more obvious, something like a seed place or hardware wallet sitting in a safe or a seed plate screwed to the underside of a couch so that it looks like someone did a quick and dirty repair?
Which I may done with a normal piece of metal I found.
Makes you wonder if someone could make a seed plate that is 'aged' to look like a piece of junk metal that has been sitting around since before the home PC existed, never-mind
BTC-Dave