I pug a lot, especially while leveling, and when I was leveling my druid (spec'd kitty, and this was before dual-spec was implemented) I often (in fact, usually) filled the tank or healer roles, meaning I had to have the gear banked away for those rolls. Usually I would only roll need on gear for the role I was filling at the time. Sometimes I would ask at the start of the run if everyone was cool with me rolling on gear for a different purpose than what I was doing (i.e. "I know I'm tanking, but is everyone alright with me rolling only on healer loot? My healing set is in need of upgrades"), which was usually cool with the group, but I generally wouldn't do it (wouldn't even ask) if it was going to have me rolling against a lot of other people in the group (i.e. rolling on caster loot when there's a resto shaman and a mage in thre group). If an item dropped that wasn't for my current role but also wasn't usable for anyone else in the group, I'd generally ask before rolling and I don't think I ever had anyone say no.

The point is, communication. In this situation, (I'm not saying Anonymous was wrong but...) if he had said, following the rolls: "OffspecTank and GLeader are both primary DPS, OffspecTank won the roll and therefore the item, any objections?" then the forum is open and the GL is really forced to either make his complaint publicly, at which point either a) an error can be corrected if the guild loot rules should give the item to the GL, or b) the GL will have to be open about taking the item he isn't entitled to, which will probably be bad for the raid and the guild, or c) the GL won't speak up, at which point reprimanding the lootmaster later for it will just make him look like an even bigger passive-aggressive douche, and more likely he'll just feel forced to let it go.

If everyone knows the rules (whatever they may be), it's either being handled right or a correction needs to be made upfront. If it's being handled right and everyone knows it, the GL will run the risk of losing a lot more than the item by trying to punish the masterlooter for enforces pre-agreed rules.