Actually, that sounds like a really good idea. I couldn't figure a way for it to work that it was possible for them to die but not likely or abusable, and you've got a great one right there. Mind if I tweak it a bit? Specifically, saying "If you fail the Will save, take Wisdom damage equal to your Wisdom score" so that there is a clear way out of the coma (I figured that the larger amount of damage makes up for it being damage instead of burn).I also think there's a problem with the fact that the nonlethal damage on awakening is incapable of killing someone, even if they have a heart condition or the like. Use of lucid dreaming as an assassination effort should be almost impossible, but not inherently defined by the system as completely so. I'd go for something like "roll the target's Hit Dice as nonlethal damage; if reduced to 0, the target fails to awaken. Make a Will save 5 minutes later to wake up; if you fail, you slip into a coma. While comatose, make a Fortitude save; on a failure you die." Something along those lines. The probability of death is extremely low, but if you spammed this effect nightly you'd have a chance of it eventually working, especially if you targeted people who appeared frail or weak-willed.
Now why didn't I think of Concentration for this?The problem is just that Bluff already has three full synergies and a conditional one attached to it; I'd be happier not having it become more of a god-skill. Perhaps Knowledge: Psionics or Concentration instead?
Defeating a Nightmare? That's a good use. How should I scale the DC to the caster level, do you think?