Well, it is possible for natives to defeat a European Style Army in this period. It happened a lot during the Seven Years War, Montcalm only having held out as long as he did via the use of his native allies.
I think to make Pontiac's War Successful, you would need butterflies in the 7 Years War. . .Perhaps Leuthen Goes Badly for the Prussians in 1757 resulting in Prussia being almost knocked out of the War. This would allow the Austrians the Upper Hand in Central Europe and force Britain to divert men to shore up Prussia.
With the New Focus on the European Theater by Britain, Wolfe has less troops to fight the French in North America, resulting in an inconclusive invasion of Quebec in 1759, the British Retreat back to the Areas they had already taken control of, which means you still have SOME French Troops running about in Quebec when Pontiac Launches his Campaign.
Pontiac would then coordinate his Attacks with Montcalm and we might see Pontiac and the French take back the Great Lakes. With Pontiac's Troops adding to the French Numbers, the War in North America might become competitive again, though I rather doubt a combined Franco-Native attack into the 13 Colonies would work.
The Question then becomes one of what happens in Europe, with Pontiac's Rebellion being just one part of the Wider 7 Years War. . .