The .410 wad does not seal the bore in a 12 ga. barrel. So there should be no possible pressure problem. I would not worry about using them unless you worry that having fun could be habit forming. If you are worried I am always willing to by an extra pair to put your mind at ease.
"I would think you could use them a good piece of bamboo fitted to a suitable breech." You might not want to use bamboo as I do not think that it has enough hoop strength for the job. Maybe we can ask Mr Bell to find out for us.
