I think they may have accepted such an idea in, say, 1766, before the conflict had grown irreconcilable with Sons of Liberty and tar and feathers and all the rest. By the 1770s, you're right, at least many colonial leaders would not have accepted such an offer.
As was pointed out, Quebec included the entire Great Lakes region at this time. But it was largely full of French and Iroquois and various Algonquian persons who would not have had much interest in any seats in Westminster, you're probably right. Nova Scotia is a different story, however. Maybe even Newfoundland and the Floridas.