I understand that there generally wasn't significant support for a revolt against the crown in British American possessions outside of the 13 colonies which did declare independence, even though the Continental Congress made overtures to several of them.
In the areas which Britain occupied after the French and Indian war, was there any change in sentiment when their former motherland joined the conflict with Britain?
In essence, indifference.
The general sentiment was that France had abandoned New-France in favor of the French West Indies, that were much more profitable than the rather onerous North American colony. This is also a sentiment that can be found in the lesser rates of volunteerism in WWI, although at that point, it's a lot more complicated.
Concerning the overture made by the American congress to french Canadian, it was more in the hope of making sure they wouldn't attack, as the French-Indian raids were still very much in the minds of the revolutionaries. To simplify, the Americans were scared of the french colonists descending by the Champlain River to greatly undermine their effort.
The other reason is basically because the British, seeing shit was about to hit the fan passed the Quebec Act which softened, if not completely reversed, a lot of the anti-catholic/ anti-french policies the British had put in place after the Conquest, making it considerably less interesting to rebel against the authorities that basically almost restored the way of life before the Conquest.
EDIT :
After going back a little bit in some reading, the American Congress did send letters to French Canadians as early as 1774, The letter title is : Lettre adressée aux Habitans de la province de Quebec, Ci-devant le Canada". It's in French, so you can definitely know it was addressed to the french speaking elite. That very same elite that benefited the most from the Quebec Act...