What about Quebec? Would the Americans be able to get Quebecois nationalists to collaborate with them?
I'll stick my foot here and say, well, no.
At this point in time, what exists is not "Québécois" nationalism per sé as we know it. It's still French-Canadian nationalism, albeit scattered throughout Canada, New England, and the Midwest with Quebec as its main centre, and among the nationalist élites the focus is on la survivance, on resistance to assimilation (even if French-Canadians themselves were happy to migrate between both worlds, although in the dominant English-speaking world they would have occupied an inferior position in the class structure). Also, keep in mind that what was French-Canadian nationalism back in the early decades of the 20th century is modern English-Canadian nationalism and the Québec federalist movement now, during the early decades of the 21st century and the last decades of the 20th century. It's not as open and shut a case as one would think, influenced by what happened during the 1960s and after IOTL. As far as US attitudes towards French-Canadians towards this period (as well as English-Canadian attitudes outside of Quebec, which at this point in time "English-Canadian" would not be the preferred term used but instead British-Canadian - as if Canadians were just mere subsets of the British people), I'd say look at historical attitudes towards Jews and current attitudes towards "Mexicans" (read: Latin Americans in general) and you've got the traditional attitudes towards French-Canadians in the US. It also didn't help that other immigrant groups - Irish-Americans, for example - were not big fans of French-Canadians because at a time when other immigrant groups were demanding higher wages and better working conditions, French-Canadians were - from the point of view of employers - quite happy to work in dangerous conditions for very little if any pay, thereby undercutting any advancement that the labour movement would have made. It's no secret why groups like the KKK and the Technocracy movement thought that French-Canadians were "inferior peoples" who they thought would be much better off if they were exterminated and that the KKK had a very significant following in the US North. Faced with this, in the US Franco-Americans had to fight every step of the way so that the community could maintain its rights, and the same was true among French-Canadians in Canada (even though in the latter case the BNA Act and its predecessor laws specifically mentioned that French-Canadian linguistic and cultural rights were to be protected).
In a Central Powers US that conquered Canada, I'll just say that the position of French-Canadians in general would be rather ambiguous. As long as they are perceived to not created much of a fuss, then they would be OK. Smaller communities, i.e. Bonnie Doon near Edmonton, AB, or the Acadians in general, would face much more of a challenge towards assimilation (as was the case in New England, where Franco-Americans were up until the last decades of the 20th century the one Euro-American ethnic group most resistant to assimilation, or still the case now in Louisiana), while the greater one's connection to Quebec the greater the chance of potential survival, although their interactions with the outside world would be tinged with suspicion, discrimination, anti-Catholicism, Francophobia, and all that pizzazz. Fortunately, one of the great strengths that allowed French Canada to survive was its ability to adapt to surrounding circumstances while preserving the core of French-Canadian identity, and this was more so the case of ordinary people, so even if French Canada - which ITTL would just simply revert back to simply "Canada" without any qualifiers - had to fight for their rights, this time within a system much less conducive to what Americans would see as "special privileges" (and even more so ITTL when the US allies itself with the CP), it would still make out OK and somewhat still in one piece. If French-Canadians ITTL were smart enough and able to forge an alliance with Native American and Métis peoples, then French Canada - as an adjunct of this - would definitely benefit from the rise of the American Indian Movement (though as a largely European-origin people it would also have to have self-reflection of its own - which would be perfectly acceptable, since historically during the colonial era there were in general much better relations between the French and Native peoples than elsewhere in North America).