Hi D Cook,
Below is a quick phone photo of the copper elbow connected to a Piano 32. A Piano 32 is identical to a Piano 26, but the 32 has a few more keys). I like the concept of large diameter tubing, but this tubing is so stiff that I cannot use it, unless I can figure out a way set its shape. For experimental purposes when testing this tubing, I did not use a mouthpiece. I just blew into the other end of the tubing.
What we need is corrugated, highly flexible, non-kinking tubing with a 3/4-inch inside diameter. With such tubing, we could further develop and prove the concept (that wicked-large tubing offer advantages in tone and expressiveness on these melodicas, which have large entrance channel and reed chamber volume).
If my initial observations of deeper tone are confirmed with the new tubing, playability does not suffer, and expressiveness increases, then it would be relatively easy to attach any mouthpiece you desired.