Server and client have a extra proxy prog that handle pak download to client and client restart. The client mod itself makes sure to not let game join until it's proxy seys paks are same as on server and downloaded) using rest api to local proxy.
// extra port needed
Note: Client -> Lua can use UE4SS to dig directly into Unreal Engine's own internal reflection system -> but runs on main thread, so downloading a PAK will probably break the system, but for quick connections to a sidecar app should be ok