regarding Quozl's match, it would suggest that the 200 pool of spell card are included within the cost of the planeswalker. Thus to give and heroscape hero the spell casting power, maybe add 200 points and give the hero a color. I feel like for any in-depth crossover, all heroscape units are going to need to be assigned a MtG color in addition to their General that they follow.
The "hex tax" idea is kinda neat but that would nerf a lot of squad based armies. Gruts would need to pay for every space used and that spells disaster for swog riders. I know it's just an idea thrown out there but it creates some subtle problems for the core game.
I like the idea of a Heroscape army starting on the field but I also like the idea of being able to now summon in a squad using a planeswalker rather than having my minions of utgar spend 3 order markers to enter the fight. -mine is an evil laugh-