I don't usually go much for fusion kind of music, but people who get both traditional music and punk rock are often the exception. The key is to "get" the core essence of each.
(That said, allow me to observe that things like bluegrass versions of Eagles songs or some similar abomninations in that vein are the cultural equivalent of the Rwandan genocide).
One band that gets it thoroughly is of course the Cankickers, good friends of mine. They play old time and punk music and are good.
Another band I just happened across is from San Antonio called Piñata Protest and they are doing a punk-conjunto style which sounds pretty good, give it a listen. I think "La Cantina" works best, hard not to agree that its good, no?
The Cankickers are a lot better, but they have been at it longer too.
It would probably be appropriate to point out that so much of early American punk rock was essentially polka music, at least in terms of the beat.