re: Evil plan
It's true that a nuke can deliever energy equal to mag 6, but it is where that energy is applied that matters most. I don't think nuking the surface of a super-volcano's cauldera would do the trick. The magma in Yellowstone is mostly at a depth of around 15 km. So the bottle cap in this case is miles thick...you'd have to use a bunker buster that could not only destroy the intergrity of the capping rock, but give the magma a good shake as well.
Furthermore, the park has experienced temblors of at least 6.4 in the last 50 years (1964?) and nothing happened but slight changes in the hydrothermal system.
(but maybe bison, with lasers attached to their heads......)