If I put a steel plate with seed phrase in bag and it goes through TSA's x-ray machines at the airport, am I at risk of them being able to read the words?
High resolution x-ray cameras of the type used for baggage scanning would likely be able to read stamped metal. For an example, this Chinese security manufacturer shows an example image where you can pretty easily make out a lot of detail. Looking at their product line shows they have a variety of x-ray technology that would be capable of producing high resolution images from metal contained in your bags.
This answer over on the Physics Stack Exchange site says:
The X ray detectors work by sending a beam of X rays through the stuff they are probing. The higher the density, the more X rays gets absorbed, the less one detects on the other side. In fact, we are not measuring density, but integrating it along the line of sight; the total mass the ray had to go through.
So assuming the stamping slightly modifies the density of the metal or the thickness of the metal, presumably anything written on a metal surface might be read.
Multisig wallets are a way around this particular security concern in that you only have an individual piece of the private key required to spend coins. This answer has some details about that. Check the list of questions tagged with multi-signature for even more info.