I see no reason midwives couldn't administer antibiotics in a birthing center. They do in the hospitals (well, the nurses administer them, but the midwives order them).
You can have what they call a hep lock or a med lock in your hand, where there's a port to hook up the IV, but you don't need a continuous drip. They can unhook it after giving the antibiotics, just leaving the little port in place for your second dose later on.
Chances are you will be GBS negative this time, but it's really not a big deal -- or, I think, too inconvenient to get the two small doses of antibiotics if necessary. I would not recommend refusing the antibiotics, though -- the chances are a baby born to an untreated GBS+ mom will be fine, but I believe GBS is the #1 infectious killer of newborns. The antibiotics will make sure your baby isn't part of that statistic, should you test positive.
Good luck!