As I read that I would have to pick fire and then use Brimstone Blast (for instance) in order to use it. Also, it can only be applied to spells, not SLAs. Also it makes it a spell level higher as part of it's cost... which is immaterial to an invocation.
I suppose one could use a "Sudden Non-Lethal Substitution" type thing which could apply 3/day as the other ones that could work... I'd really like to make it so that anytime I used it it could be Non-Lethal...
Perhaps it would be best to make it an invocation, very much like Hellrime or Brimstone Blast, but Nonlethal Blast. Could be nice for an Exalted Warlock.