While there may be some effect from isostatic rebound, there is a chicken/egg question. Volcanic activity will be preceded by higher heat flow at the surface. Once started, ash deposits will decrease the albedo of existing ice/snow fields and accelerate melting, provided atmospherically suspended ash is not significant enough to retard insolation. Certainly, melting will accompany volcanic activity. To say melting owuld cause volcanic activity is a far stretch when there is the entire Canadian Shield in isostatic rebound from the last Ice Age and recent non-margin volcanic activity is pretty small.
The kimberlites in AB, for example, are 45 to 75 million years old, whereas the Caldera activity in Yellowstone is a few hundred thousand years old, and the Long Valley Caldera only a few hundred.
If melting ice caused volcanoes, (instead of the other way around) Saskatchewan, Manitoba, North Dakota, and Minnesota should be prime for eruptions--the ice sheets here were well over a kilometer thick, and in many places three times that.
Where melting ice and volcanoes are associated at all, my money is on heat flow from the magma melting the ice, not the melting ice causing volcanism.
The Long Valley caldera activity was over 700,000 year ago, and the last major Yellowstone was about 640,000. However, Yellowstone has been blowing its top with major excess for 16 or 18 million years, as the magma plume has advanced to the east under the continent. National Geographic had a detailed article on this not too long ago with excellent maps and drawings.