The machine shop near me that has been building race engines since the 1950's uses Brass freeze plugs and they use some sort of a shellac to seal them in. Kind of looks like that Indian Head stuff by permatex. Whatever it is, I know they use the same stuff around their cam bearings when they install the cam bearings.
My father had a brass freeze plug fail on him... in fact two at the same time on his 1948 ford coupe with a sbc 307. He couldn't really tell me what happened until he noticed the temp was going through the roof and looked behind himself at the coolant trail. He didn't build the motor and isn't much of a mechanic. So, I suspect it was due to an over heating issue, coupled with pressure. Blew both of them clean out of the block.
I took a look at the block when I was in town after that. I did notice some corrosion around the holes on the block. May have been possibly due to a bad sealing surface.
Make sure you clean the holes up before installing the plugs. Sand them smooth or wire brush them or whatever, then get the grease, oil, and other junk out of it before using any sort of sealant and driving them home.