I guess I've answered my own question somewhat.

If the original song I play is in G on a C box, I could then get a G box, play it in D, and the fiddler should have little problem playing on a standard tuned fiddle. The strings primarily played will be the middle two, rather than the upper two. To me that would not be a problem, because I don't go all over the fingerboard when I play. To some it might limit their playing.