My high ranking Tribunal Temple Zealot wants to wear Indoril Helmet for certain "ordination" missions, which can be bought for hefty 3000 gold at Ordinators Barracks in Ghostgate - an amount next to impossible to save for a poor pilgrim, but for a good cause like this he could (finally sell some loot out of his RP range). However, I've read somewhere that Ordinators attack characters wearing Indoril armor parts. Would they really attack my temple fanatic wearing a legally bought helmet and otherwise having very high disposition towards him? Or there are only certain occasions and conditions when they attack for such a crime?

There is a script on Indoril helmets and cuirasses which causes ordinators to hunt you down if you equip them regardless of situations.
Best not to be seen by ordinators while equipping them.

Seen or talked to? I experimented and bought one and equipped it, but the Ordinator standing next to me didn't attack me until I talked to him. Or he (and all the Ordinators) got a script to attack me already by just seeing me with that helmet? I really wouldn't want to go wrong with this one. I mean. The helmet is legally for sale.

As long as you don't talk to them, you should be fine.

If an Ordinator sees me with that helmet but I don't talk to him, then I unequip it and talk to him, his disposition won't be ruined by what he saw before?

EDIT: Yep, that seems to be the case. I can dance striptease with the tune 'You Can Leave Your Indoril Hat On' in front of an Ordinator, then take it off and talk to him and he still likes me very much (who wouldn't).

Thanks guys.