My idea is simple:
With a "No graphic" helmet i can assign the VFX property into the "onequip event" in order to show the correct graphic (for example a circlet); but since it's a true helmet i can place the properties on it without any problem or additional script to keep the bonuses active as effects.
Then i place the VFX removal on the unequip (it's possibile, with a NWNX plugin, to remove a specific VFX).
The selection of the vfx can be done with a simple TAG or RESREF parser
For that i need an helmet that leave the original head untouched (like a cloak with no real cloak model), so that i can apply the VFX on it.
I'm using CEP 2.4 for helmets, you can override helm_118 if you need, or even one of the NWN standard helmets (except the first used on the "Pot Helmet", since it's the only one i use)