The modern banjo came to form in the 1860's-90's. What age is that? That's right, that's the STEAM FUCKING AGE. In other words, the banjo * ALREADY IS * a steam-age instrument. In fact, it's better than Steampunk because it actually dates from that age and actually got built.
Want to see what a FOR-REAL steam age banjo looks like? See these. They look like banjos.
(No offense to the Steampunk folks, but sometimes the real thing is just better.)