Spain captures Georgia, from the British, which was a small, new settlement did not have any slaves. But seeing the British beaten on the south side of the river, inspires the numerous slaves on the north side of the river in South Carolina to rebel? They likely escape in addition to rebelling, settling in captured Georgia or Spanish Florida?
Or, having conquered Georgia, the Spanish keep going, crossing into South Carolina, and their direct presence or arming of captured plantation workforces sparks the slave rebellion?
OTL, the Spanish promoted slave escape, promising freedom and land to those who made it across the border. A small number took up this challenge. They formed a small community in the vicinity of present day Jacksonville, Florida. They either fled, or were dispersed when Britain took Florida after the 7YW. Georgia did not have much, if any, slave presence at the time, so the slaves must have come from further north. There was widespread concern/panic that the movement could pick up steam and the east coast would be in trouble.
My thought is that, seeing Spanish success in Georgia, slaves in the Carolinas, or even further north, rise up in larger numbers. You could write a scenario where Spain decides to concentrate on the colonial War of Jenkin's Ear, instead of switching theaters to Italy. I don't think such a scenario is likely, but, presto, magic hand wavium, it is so. Britain's navy was pretty battered from tropical rot/disease, so there is a window of opportunity for Spain to send resources to the New World. IF the slave uprising is disruptive enough, maybe Spain crosses the border into South Carolina. Expecting some far reaching expedition north is asking a bit much, but, hey, maybe the winds of war can fan the flames of slave rebellion up the seaboard. As noted, this was a fear, OTL.