Do it subtly just like you said. I am seeing kind of a rockabilly elegant wedding and not so much the cheesy elvis or death metal rock wedding.
Emphasise circles and add touches of records through out the reception…for instance instead of the usual mirroed tile get a bunch of old records from the thrift shop and use them under a few candles or the flowers. You can also heat them in the oven on top of a large ceramic charger and they will melt to form that shape.
Try and find it in the budget to get a live band that does the old rockabilly style.
Choose a really retro, energetic color scheme like black, white, aqua, and cherry red.
Name the tables after famous rockers and include a brief bio about each one
Add touches of musical notes and old electric guitars as images on the stationary.
Perhaps give out the big rocker aviator sunglasses as a fun favor for guests to wear during the reception.