Is Morgan's Earth Stomp actually legal by the rules?
The idea of attacking against Might kinda bugs me, and as far as I know it's never mentioned outside of that example.
So...yeah. Does anyone know if there's something in the spellcasting rules that allows this?
If not, then why should it be possible with magic when it isn't possible without magic?
Working from the rules in the Evocation section, there isn't an option to change the Skill with which someone defends. It points you to the regular combat section, which mentions Athletics, Fists, and Weapons (naming Athletics as your catch-all defensive Skill).
Might lacks a defensive trapping outside of wrestling, in which case it can replace Fists; this isn't explicit, but presumably it can replace the defensive trapping of Fists in this instance. Other than that, no defensive trapping.
So far as I can tell while doing this reading, the Might part doesn't adhere to the rules. Either it's a Block against which you need to push, in which case you use Might. Or it's a physical attack, in which case you can use any applicable Skill. There's no rule to force someone to use a particular Skill to defend; likewise there's no rule to even allow them to use Might to defend at all in the first place.