"Because the top barrel is holed it will have to be sleeved but the bottom barrel could work with a liner or insert."

Yes, but no need to reinvent the wheel. If job is worth doing, it is worth doing correctly, nicely, and your final product will be worth more in the end, meaning I strongly suggest that you forget about lining any barrel. Cut both barrels off, forming a nice compact monoblock, bore & ream that monoblock to fit in pair of rifle barrels. Brown and Ron Villa thread part or all of the monoblock and barrels to screw and soft solder them in place. That threading isn't necessary, as simple soldering alone will work quite well, especially for something like a .22 hornet. Again, get W. Ellis Brown's book on conversions, 2nd edition only for updated version of book (it is available online). That will answer your regulation questions and many others as well. Also, do search of Ron Villa's postings on his conversions, on this website, as he shows step by step pictures of the operations for conversion. Ron knows what he is doing, obviously, and I highly recommend paying attention to what he has said and done regarding this topic.