If the pressure cap on the upper tank (what you would normally refer to as "the radiator cap") had a weak spring or bad inner gasket, you could also fill up the "puke tank" without a serious overheat, just a little hotter than normal could do it. Monitor the coolant level closely.

The puke tank can be removed without removing the entire fender, just the liner and (preferrably) the rf wheel.

Gilly