Well, since it says
you can use an immediate action to either cast a spell or make a single attack.
I think it's pretty clear that you have to use your own personal spell slots. This is still quite good, as it would allow you to, say, shapechange into solars earlier, with the diehard feat, or get greater magic weapons up to +5 sooner, or cast more mirror images or something. Note it says reduced to -1 or fewer, not actually knocked out. You can just get someone to throw small shuriken at you while you have fast healing 1 from vigor or something, and lo and behold, you have free caster levels.
EDIT: Ah, I see what you're saying. You mean you could cast a spell with a long casting time. Well, that's certainly a nice trick. I think that that would work, indeed.