Originally Posted by Crisischild
As far as I know only smite is unaffected by damage amplifiers be they abilities or masteries.
This is incorrect.
Things like this are technically coded on the ability level, but not in the way you say:
Smite says, "Deals X True damage to target." That's how it's coded and that's how it functions.
Something like Hemoplague says, "Take any physical or magical damage that is coming into this target. Multiply it by 1.12"
Some abilities may be out of date in that they say, "Take ANY damage and multiply it" and so those may still amplify true damage, but the design intent is that the number in the tooltip is always the value dealt.