Steve,
I've done this several times. I buy the three-packs of ribs at Sam's club and put a different rub on each. I take three wooden skewers and notch the ends with a knife, one doesn't get notched, one gets one notch, the last 2 notches. Make sure you spread the notches a bit, as they tend to get crap all over them, making it hard to determine which one is which. Roll up the racks and stick whichever skewer you want through them, taking a moment to write down which notch number corresponds with which rub. The skewers make it easy to tell what you're tasting later and you don't have to keep track of them on the grate.