Warrior spells in general tend to need to have correct Stance active (so learning them alone won't give you possibility to cast them, but Overpower doesn't seem to have Stance-dependencies). They also cost rage. So if your character has basicaly 0 rage while its not a Warrior and has no Stance active while it (probably) doesn't even know any, how can it possibly cast Overpower (or any other Warrior spell)? Spell isn't broken and I don't believe that there is any "class restriction".
I would suggest you to try to learn some Stance ablitty and/or use it and also to use .mod rage 100 on you. And how to make this accessible to players? Well, the best way I can imagine is creating duplicite spells without any cost, or with mana/energy/runic power/runes costs for certain classes so they can use it.
