Sable Island I think would be your safest bet in Eastern canada, because it is far from the continental landmass, and therefore its weather is indeed more moderate. Winter is still colder and longer on Sable Island than it is along the coast of BC (and especially the south coast) but I bet one could plant windmill palms there and have some success (if not cared for in the winter I bet they would survive, but maybe more in a stunted shrub form)
As for Yarmouth, I believe it would face similar limitations as the south Okanagan in the interior of BC and extreme southern Ontario, where one would have to perform intensive care in the winter in order to keep a permanently planted palm alive, and even then it would be risky. Yarmouth still haves 2 months of the year where the average low is -7C and -6.8C. These lows are actually colder than that of a few spots in the south Okanagan in the interior of BC.
To give examples of the difference in winter weather from southern Nova Scotia, which has the mildest mid winter temps in canada outside of BC, to BC, here are a few January stats.
Sable Island, NS = H
2.8, L
-3.5
Yarmouth, NS = H
1, L
-7
Vancouver airport BC = H
6.1, L
0.5
Vancouver Harbour BC = H
6.8, L
2.7
Victoria airport BC = H
6.9, L
0.7
Victoria Phyllis Street BC = H
7.2, L
3.7 (This station is near Oak Bay, where many of the largest, most varied and lush palm trees planted in BC grow. The coldest temperature ever recorded at this station in January is -8, the coldest temperature ever recorded at this station is -11, once in February 1986. For a fun comparison, Las Vegas, Nevada's record low is -13.3)
Tofino BC = H
7.6, L
1.4
Richmond BC = H
6.6, L
0.2
Amphitrite Point BC = H
8.4, L
3.9
Nanaimo BC = H
6.2, L
-0.8
Powell River BC = H
6.2, L
1.7
So as you can see winter temps along coastal BC are far warmer than that of Yarmouth, Nova Scotia, and decently warmer than that of Sable Island, Nova Scotia. (Most stations along the south coast of BC are 6 to 9 degrees C warmer than Yarmouth in the dead of winter, which is very large in climatic terms). This is what really gives coastal BC the edge in permanently planting many exotics in Canada.
Again, Sable Island would be Eastern Canada's best chance (since its climate does definitely have its own, smaller edge over the rest of Eastern canada), the only problem is that Sable Island only has a population of 5 people and it is protected by the Federal Government and one has to obtain permission to visit the island, so i do not think there is much gardening occurring on the Island.