Is it safe to cook or braise with beer in cast iron? Is it safe to cook with beer in cast iron, for longer cooking such as braising or stews (eg, cook over 30 minutes, probably for 2 hours or more)? I saw on this question, Is it safe to cook wine or vinegar in cast iron?, where they pointed out wine or vinegar should not go into cast iron for long cooking periods, because the acidity would alter the flavor, or worse could leech an unsafe amount of iron into the food. 

On further searching, the above question noted wine was around 3.4 pH, and I found on other sites the pH of beer is around 3 -4 pH. So, would beer thus be included in liquids to avoid in cast iron?

This is for regular cast iron, not enameled, and with amateur seasoning (by which I mean, I used some lard once at 500 deg for one hour, and later some coconut oil, and deep frying, so probably not a great protective seasoning.)
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