If you play WHM correctly you're already a pretty HoT oriented healer. Regen is your absolute bread & butter spell, more-so than even basic Cure. Then you have Medica II. And finally, there's also Asylum.

Sure, we're not like a Druid in WoW (which is/was primarily a HoT based healing class, at least last time I've played), but a very large part of our standard healing is already done over time.