The Mischiefmaker PrC (Specialty Cleric of Erevan Ilesere.)

[CopperElfCleric]

It's all good!
The problem is, that ruling means that either:
a) Surprise Spells stacks with the default sneak attack damage, dealing a whole lot of damage output on weaponlike spells.
b) Surprise Spells doesn't stack, so only AoE spells are affected.
c) You could errata out those rules.

As an alternative but similar effect, maybe something akin to a beguiler's sneak-attack-adds-to-save-DCs effect?

I would either errata out those rules or have SS stack with default sneak attack damage. It's only 1/day anyway. Not overpowered imo.