I'd say 'all' can only be followed by a singular verb when the meaning of 'all' is 'everything' -- and that is probably limited to well-known expressions most of the time:
All is (not) lost. All is forgiven. All is fair in love and war. All's well that ends well. All is calm, all is bright.
In the case of "All I want for Christmas is my two front teeth." I am inclined to argue that it is wrong. In this case. it would be "The only things" I want for Christmas are my two front teeth.