Well... My scenario played as such:
The War of Austrian Succession having the Spanish defeating the Dutch in the East India Company, forcing a relatively large number of Dutch settlers from their colonies.
The first problem here is there weren't a large number of Dutch settlers in the Indies; there weren't any at all in the 1740s- even in the 19th C there were only scattered plantation owners. In the 18thC the Dutch East India Company was like the British East India Company- forts, traders, and soldiers.
These Dutch try to flee but end up taken by the currents into North America, where they settle on Northern California/Southern Oregon.
European ships of the 18th C weren't taken anywhere by the currents, and their navigators had a reasonable idea of where they were going. If they had already done some exploring of the Pacific Coast- and learned about the value of sea otters to China- they might have headed there deliberately, hoping to get north of Spanish territory in Mexico.
Isolated, they, in the following years, enter into several conflicts with Indian tribes and the growing Spanish presence in California, resulting in a Great Trek that lands them on Eastern Louisiana, where they rapidly set foot, gaining notoriety as they have a "great awakening" of sorts where they develop a recalcitrant calvinism, which also attracts a few Huguenots.
There wasn't any Spanish presence in Alta California at the time; San Diego was the first fort/mission, established in 1769; Spanish overland exploratory expeditions didn't reach San Francisco until 1775.
Never mind launching a trek across the Rockies and into Louisiana Territory.
Not trying to be a downer, and I'm not one who says "all ATLs are virtually ASB" - my New Albion TL in the same time and place stretches the limits- but you're going to have to find a different method of getting enough settlers to the coast. Somehow have the Spanish chase enough Boers out of the Cape; first to the Indies and then to the West Coast?