Thing is, where would enslaved African-Americans have been exposed to Celtic wedding ritual traditions? Correct me if I'm wrong, I could well be, but I was unaware of any widespread presence of Celtic traditions in 16th-19th-century American culture.

If that isn't the case, and if we saw traditions similar to broom-jumping in both Africa and among Celts, it would seem to be more likely that enslaved Africans in the US would have brought those traditions over with them across the seas than it would be for the traditions to be introduced to them once here.