I would start the ganache by reducing the stout to a syrup consistency
Does your experience give you any idea how much of a reduction that is? 75%? Do you think that would concentrate the bitterness too much, or will it be offset by the sugar? Or would you start with something with low bitterness?
Thanks for the book recommendation 
Heres the ganache formulation I use, its from Grewelings book
250g Heavy Cream
70g Glucose Syrup
60g Liqueur or liquid flavoring (beer reduction)
30g Butter
620g Dark Chocolate
I would start out with at least a 50% reduction and maybe as much as 75%, just a guess. I'll keep you updated when I develop my recipe, I'll have to do some trial and error. The bitterness shouldn't be a huge factor since you're not using a large amount of the beer plus it should be offset by the sweetness of the glucose and chocolate (provided its not a high% dark chocolate), If its a highly bitter beer maybe try using a milk chocolate to make the ganache to balance it out.