You don't need Swift to use the Keychain API. It's doable from pure C.
Good to know, my understanding of the macOS system APIs is fairly limited. I'm sure it's doable, with some elbow grease and CGO. We just haven't prioritized that variant of the client due to relatively low usage.
In fact, SecurityFramework doesn’t have a real Swift/Obj-C API. The relevant functions are all direct bindings to C ABIs (just with wrappers around the CoreFoundation types).