Can I ask why homebirthing is definately out if you have screened positive? Are you with an independent midwife or a 'low risk' government program or something? If you haven't had GBS before, you're low risk, I can't see why there is a dilemma if you genuinely want a homebirth. Observe the baby carefully after it's born if that will put your mind at ease. Keep risk in perspective. You or you child could be killed in a car accident tomorrow, but we still get in the car don't we?
HTH.