That's an interesting question. I have tried using Bullseye frit on copper with little success unless I do a layer of Thompson enamel first. And, in order to enamel on sterling, you'd need to depletion gild it first to bring a layer of fine silver to the surface (so I skip this step by working only with fine silver).
Now, I'm wondering if Bullseye might react differently to silver than copper? I'll dig out some scraps and play with it. I love the Bullseye color palate and it would be a great way to use scraps from larger fusing projects.