I did this off 1 bottle so the samples were pretty small and I just added a few drops of each liquor. I was surprised by how much that small amount of a liquor can completely change the character of the beer.
Here are the results in order of tasting:
Plain: chocolate, soy, a hint of alcohol, moderate bitterness
Makers Mark: Alcohol, wood, marshmallow
Godiva: chocolate, smooth, coffee
Frangelico: Nutty, wood, chocolate
B & B: Herbal/mint, fruity, bitterer
Kahlua: Coffee, roast, bitterer
Jagermiester: Licorice, muddy, herbal
Chamboard: Raspberry, chocolate, non-complex (dull)
Remy Martin Grand Cru: Booze, concord grapes, coffee
Malibu: Tropical/coconut, lacking, chocolate
Clearly I'll have to make a Nutella (Frangelico/Godiva) sweet stout at some point. I may also take to making my own "bourbon barrel aged" beers from time to time.
More Liquors will be added as I try them out.