It would look something like this maybe:
/assist focus
/castsequence reset=combat/target Curse of Agony, Corruption, !Shoot

.... I think that would work .... can't test from where I am... but then you lose your sticky targeting for cc purposes (and you already can't use focus), so that may not be worth it...

Maybe more like:
/castsequence [target=focustarget] reset=combat/6 Curse of Agony, Corruption, !Shoot
would be better?

Play with the "6" till you find a resonable amount of time that doesn't reset when you are spamming on a target and want to wand, but does reset in the time it takes you to pick a new target and get in position