Unless they have changed the rules recently you are allowed to wear a t.shirt. We were there last July and lots of people wear them, it stops your back and shoulders getting burnt as well as covering up some of the bits you don't want others to see.
I think the only rule is no metal bits sticking out like rivets as these could catch on things when you're on the rides. That applies to bikinis, swimming costumes, shorts etc as well. T-shirts should be fine.