Yes these rebellions certainly are not on the level of something like the American Revolution or French Revolution (hence why they are historical footnotes) and the Upper Canada Rebellion was pretty pathetic in and of itself (note the story of the battle in Toronto in my first post) but they do serve as an important launchpad for a US mounted invasion. Firstly they would give the US government a way to rally public support (to free fellow colonials fighting against British tyranny) that they didn't have the previous times they invaded BNA. Secondly they would have local allies in the Canadas that would support them (not just the Rebels themselves but a good chunk of the populations, particularly in Lower Canada) which had previously been almost entirely hostile to the US invading forces. Upper Canada probably would have less support for the rebels (and hence more support for the British regulars and colonial militia) than Lower Canada but it is also a lot more vulnerable to US invasion (much less fortified and less populated then Lower Canada at this time) and one would think that with the British forces in Lower Canada fighting the much larger rebel forces there that an early American invasion could sweep through the South Ontario Peninsula fairly easily. I would also argue that a direct American military intervention would cause a significant change in the nature and makeup of the rebellion, particularly in Lower Canada where a large portion of the Francophone population sympathized with the Patriotes but didn't actively join them. With the US actively joining the fight on the Patriot side you would think they would be able to draw much more material support form the populous than in OTL. Of course an American invasion would likely have the reverse effect on some of the populous there and push them to side with the British, but this too would definitely lead to an escalation of the conflict (which is the point of this thread).
Small note, the majority of the Francophone population in 1837 did not sympathize with the rebels. The sympathized with the Church, who followed the British lead.
The point I'm trying to make is that to get the widespread support on the ground which didn't exist historically is that you need to change the basic makeup and message of the rebels themselves.
To do that you at least need a POD in 1815.
Though to get a more willing US you probably need one in 1814.