even though it's not recommended it really seems to not make sense.
When you think about it, the brass cartridge case absorbs a lot of the pressure. For example if a case defect blows out in a smokeless gun, the chamber can blow out too. I know the frames do stretch in brass frame, but you'd think it would be worse with loose powder. Again, not saying to try a conversion cylinder in a brass frame. I think a lot of it has to do w/ lawyers. Somebody will sooner or later put a full house load thru one of those cylinders and a brass frame definitely can't handle that.
I would be interested to see someone test pressures... maybe 15 grains FFFg in a 51 navy, vs. 15 grains FFFg in a 38 S&W conversion cylinder. If it would fit in that little cartridge. Well you should at least do 10 (never tried the 38 S&W so I don't know its capacity without looking it up).