The Testament of Exorcism book (available from Milton Sheaf in the Stormwind Library) states that “Exorcism can now be cast on any target”. This has worked for most targets, but the Black Broodlings in Burning Steppes report “invalid target”.
I haven’t tried it on other dragonkin.
Edit: Nothing to do with dragonkin at all. I’d hit level 52 and had bought Rank 5 Exorcism. Rank 4 - the rank I had when I used the book - can hit all targets, but Rank 5 can’t. Oddly, although quitting and relaunching the game didn’t help, hearthing to Ironforge and then jumping on a flight path* did: Rank 5 is now showing as working on all targets.
*I’m not sure whether it was the hearth or the flight that did it. Or maybe it was the mere action of clicking “Show all spell ranks” in the spellbook.