"I'm sure that a relatively moderate pressure round such as 30-30 can be safely sleeved "

I am with you on this but whenever possible the sleeve have to be screwed and not only soft soldered. its a big plus in my eyes but other may think different about it. small pistol cartridges like 32-20 or the little 22 lr is another thing, of course.

good you can still do such works by yourself in NZ, here its for gunmakers only.