Nope, there is no way.
The targets allowed only changes which units you are able to cast it on.
You could trigger it though. Use some buffing spell of sorts, then add a spell book containing permanent immolation, then disable it. Then run a periodic trigger (or other method eg local unit) to see if the unit still has the buff, and if it doesn't then remove the spell book from the unit.
I don't think I'm dealing with quite the same pressures though, at the very least its a significantly smaller system. For the time being I'm just going to put together a quick scrubby box though and hope it works good enough to not make my house toxic
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.