I actually called the soap base manufacturer, and they said it was fine to add at the same usage rate as a fragrance oil (but not both at max percentage rates), you just had to let it dry longer before wrapping.
Beer being liquid I would imagine that you can add up to 1 Tbsp per pound of soap. When heating / melting your soap, if you let it evaporate a little longer, you may be able to add a little more.
I would not recommend for mp.. I was going to try this however I backed off because the beer has to go flat for a few days first and since it is alcohol uncertain how it will react.
Not all soap bases are made with water and whether you use the stove or microwave method you come out better buying a beer fragrance oil instead.