I'm fermenting in a corny right now. We added fermcap-s to the boil (10 gallon batch) and I filled my corny as high as I thought was going to be safe (and then added a little more)... Nothing has come through the airlock other than excess CO2...
When I picked up the corny from the LHBS, and told him I wanted to ferment/age in it, he gave me the options for using it that way. We removed the pin style release valve from the lid, so that I can install an airlock there. Next time I get another corny for fermenting, I'll not remove the relief valve, but get tubing to go over the lock valve for an airlock. That way, I'll have more option for using the keg later. That way, I can have it sealed, hit it with some gas to make sure the lid is seated fully, then release the gas, and install the airlock.
My only semi-concern is standing stability of the corny compared with a carboy. I might look to either make, or buy, something that will give it a slightly wider base for when I'm fermenting in it, and there's an airlock on it. At least, when it's not close enough to other things to keep it from being knocked over. Although it would take a decent amount of effort, and carelessness, to tip it over.
Of course, I did remove the dip tube from the keg, installing another gas tube in it's place, replacing the o-rings on both posts.
So far, I would say that if you use fermcap-s with the corny, need for a blow-off tube is eliminated. Just install an airlock either on one of the posts (pull the dip tube so you have more room for wort
) and let it ferment.
I do, kind-of, miss not being able to see what's going on inside the primary, but I'll get over it. Knowing that I'm fermenting in a completely light tight vessel is worth it. I know that the plastic buckets are not 100% light tight, the corny is.
BTW, I'm using a 5 gallon corny here... We split the batch from Saturday so we can each ferment it at our own place. We do plan on a bottle swap once finished, to see if they are the same or not. The only difference should be the fermentation temperatures. Everything else is identical.