Don't do it. Don't past non-request-scoped data through contexts, and none of these are request-scoped. You can trivially pass such values through middlewares using methods or closures.

Also, while I think I know what "unlike Rust, these phantom types appear at runtime in Go" is saying, it's saying it in a really confusing way. These types do not "appear at runtime"; they are exactly like all other types, constructed at compile time and visible when interface-boxed at runtime. They're in no way "spectral".