An xprv encoded in Base58Check will be either 111 or 112 characters in length. The cryptosteel capsule can hold up to 123 tiles, so should be able to hold an xprv provided OP gets enough tiles. As I understand it, each cryptosteel only comes with enough tiles to guarantee ASCII strings up to 55 characters.

I still think it's unnecessary though, and duplicating his seed phrase across two capsules is the better option.