Lomotil doesn't have a particularly strong interaction with alcohol. There shouldn't be a problem having a beer. Flagyl sometimes interacts very strongly with alcohol, but you didn't ask about that.
I take Lomotil at bedtime and before getting on a boat, and that generally completely eliminates the most inconvenient bathroom visits/needs (while sleeping or far away from decent bathrooms). I would never use it to change the consistency of my stool - psyllium does a better job for me with that.