Is it safe to cook or braise with beer in cast iron?
It really depends on the level of seasoning of your cast iron. If you see a really nice sheen all throughout your pan, I'd say you are good to go. The acidity of tomatoes, beer, wine, etc is problematic when the acid reacts directly with the iron. However, the seasoning on a well seasoned cast iron pan will act as a layer between the iron and the acid.

Another factor is the length of time the acidic food is being cooked in the cast iron. With a well seasoned pan, a 2 hour braise with an acidic liquid such as beer shouldn't be a problem. 4-8 hour cooks I would probably avoid.

One thing to consider is as your braise goes on, the pH of the liquid will rise (become less acidic) as the pH neutral water expelled from the meat mixes with the beer. This is to your advantage and will allow for longer cooks.

If your seasoning is not very good, you will end up with metallic tasting food which is obviously undesirable.

In summary, with a good seasoning you should be fine. It sounds to me like your pan might be seasoned well enough. If there is a sheen, I'd say your good to go.

Sources:


http://www.thekitchn.com/5-myths-of-cast-iron-cookware-206831
http://lifehacker.com/its-okay-to-cook-acidic-dishes-in-cast-iron-and-other-1772555109