With limited testing using just macro buttons, rather than the click box healer addon, you seem to need a slight pause between the "stopcasting" click and the "heal" click, so not sure the down/up thing would work, without holding the down for the required duration and then releasing, which is just going to be forgotten in the heat of battle when you really do need that heal

Thanks for the suggestion but, for now, I've just stopped the cleric DPSing, except to start wanding - this does mean that on quite a few rifts/invasions, the cleric doesn't get any rewards, as he never gets to damage a target, which seems extremely unfair for healers