t-shirt is fine. Boxer shorts are not.
It is highly unlikely that you will get an extra pyjama set. These are strictly controlled in the inventory list of the crew.
Interesting you say this, occasionally at the end of a flight I ask for a second pair for a family member - sometimes the answer is a yes, I'll come right back, sometimes they have to check and more often than not say no.
I wouldn't have thought they would be controlled that strictly - i.e. they must have spares for spillage etc. Evidently not!