That's already possible, it's the whole idea behind CVM and Relatr. You can easily integrate it into any Nostr client without needing a service website. All you have to do is call the service using its pubkey and perform a search or look up an individual pubkey. So, if a Nostr client integrates this feature, users could specify their own instance for a personalized Web of Trust computation, or use the default Relatr service, or any other public instance. The only difference is the pubkey of the service. I've already heard from people who are implementing it in their clients.