Portugal is your best bet. In OTL, prior to Napoleon's betrayal, France and Spain planned to divide Portugal up into two puppet states. The Portuguese government, which fled to Brazil in OTL during the Napoleonic Wars, would probably stay there in a Napoleonic Victory scenario. IIRC, the excellent "Pax Napoleonica" timeline by Zach has this exact scenario happening, with Brazil later becoming a strong British ally against France in the late 19th century.
A republican revolution in Spain or Britain might force the monarchy to flee to the Americas (probably to Veracruz or Philadelphia*, respectively)