...fitting a barrel of a M115 howitzer to the M110 chassis - could that work?
Yes, the barrel from an M115 Long Tom from AFV Club can be swapped out to backdate the M110A2 to an M110. They had the same barrel.
...wouldn't a piece of aluminum or brass tubing work? Then just put a canvas dust cover on the end.
No, the barrel is tapered and flared, so a straight piece of tubing won't work.