Before I gear up Venomage for my Demon Lord team, considering that his damage would come more from poisons and masteries than attacks, would it be perfectly fine to have him on a HP%/DEF% chestplate and gauntlets - or should I be looking to start moving toward CR% and ATK%?
It depends. The top two most important stats for Venomage are ACC, so you can land the poisons and debuffs, & Speed, so you can ensure the max number of poisons up at all times. After that I would concentrate on survivability... dead champions do 0 damage.
I don't really think that's the right way of looking at it.
For champs who do not scale on either HP or DEF, you want to put as little as possible into those stats. Your goal is to ensure that nobody dies until the stun kills someone. That likely is far lower than you'd otherwise expect.
So, the short answer is, you probably should move towards ATK and CR, yes.
