I don't know that I could bring myself to do that to a Guiness but I've never tried it so I really can't say.

As for ice cream and alcohol, I know that works! In the Canadian Navy we used to have "Moose Milk" every Sunday morning when we were in a foreign port (to help with the recovery from Saturday night). It consisted of 1qt. chocolate ice cream, 1qt. vanilla ice cream, a bottle of Kaluha, a bottle of rum (and whatever clear alcohol you felt like using). The alcohol encouraged the ice cream to melt and it made a milkshake to wake up to.