This was kinda the case on Last sundays ZA raid, when Kersh was asked to respec to tank it, and tank loot dropped. It was resolved that he should get the loot for making the raid work, continually respeccing when needed and it was a vast improvement.
What happens though if you choose to raid as off spec, cos if a holy pally chose to raid as retri and rolled healer gear as on-spec and won it, i'd say i'd be a bit miffed.
Similarly if another retri pally lost out because that pally rolled on-spec for retri loot, i'd imagine they'd be pissed.
^^Note the words Chose and Choose, being asked is different^^
Do we need to define a third layer of rollage here to clarify any probs which may arise, especially as it could happen more in 25 man content? such as:
On-spec rolls
Re-Specced rolls
Off-spec rolls