Can Ionic Spark proc more than once with area on-hit attacks?
From what I have done and seen, and from what I have read, it does indeed proc multple times when striking multiple enemies with AoE attacks that proc on-hit effects. So Shyvana's Twin Bite in her dragon form would instantly proc twice if you strike four targets with it, four times if you manage to hit eight. Same with Spray and Pray.
(P.S. The chain lightning effect also appears to cause spellvamp.)