Minecraft Java Edition – How to Detect When a Player Holds a Tagged Item with NBT Data

This answer was written prior to the release of 1.13 and "The Flattening" that came with it. An updated answer can be found here.

You're actually pretty close, and definitely on the right track. Instead of {Inventory:[{id:<block ID>,data:<data value>}]}, use {Inventory:[{id:<block ID>,Damage:<data value>s}]}. The s after the data value is important since it's stored as a short, not as an int. (As an aside, if it were stored as a byte, you would append a b instead.)

Create scoreboard wSkull: scoreboard objectives add wSkull dummy

[1]Repeating command block: scoreboard players set @a wSkull 1 {Inventory:[{Slot:103b,id:"minecraft:skull",tag:{display:{Name:"Leafy Headdress",Lore:["[Rare] [Quest Item]","Some irritating thorns poke out of it."]}}}]}

[2]Repeating command block: execute @a[score_wSkull_min=1] ~ ~ ~ [command you want to happen when the custom head was worn]

At [2]Repeating command block, you can do multiple of these if you want alot of things to happen whoever wears your head.

Slot:103b locates where the helmet should be put when you want to wear it inside your inventory.