Absolutely. Will it run like that forever?? Not a chance. Eventually the engine will use all the fuel in the fuel line and will die. But in normal operations of a RC plane the time it is in a down line isn't long enough to drain the fuel completely out of the fuel line. If you're having real world issues with running out of fuel in a down line try adding a loop of fuel line between your carb and the fuel tank. The extra line will keep it running longer.

Ken