Well, Fredericks brother Henry was briefly offered the throne of a hypothetical American Kingdom/Empire IOTL, but the offer was rescinded before he could reply. There are ways that do not involve getting Frederick into the British line of succession.
In 1730, as an idealistic young prince, he attempted to escape from his abusive father, to Britain.
In my own TL, I take this as the POD, and have him end up in America... where he becomes a leading rigure in the revolution - arguably more prominent than Washington. I have him becoming an ardent republican, but that's a pretty random development.
A victorious general from the revolutionary war, who also happens to be of royal blood? That would make him a shoe-in for the role of king.