That's a great idea. For a while, I too was bound and determined to have a sliding mouthpiece. In fact, I created and tested a few types of sliding mouthpieces a while back. I'm almost certain that I sent one of my prototypes to Ubizmo for testing and feedback. (Ubizmo has given me a lot of wise, thoughtful advice and feedback through the years!)
After testing and carefully considering a sliding mouthpiece versus a "solid-state" mouthpiece, here's what we decided. (I include all the detail so you can follow our thinking on this.)
After some practice, we found that switching chambers on Coda's present solid-state design was just as fast as on the slide design. Seriously! The slide was faster at first, but it came with several drawbacks. Since switching chambers on the solid-state Coda will become quite automatic, a slide mouthpiece seemed more like an unneeded crutch that you'd be better off without in the long run.
Also, the solid-state was more portable and much less fragile. Since everyday carry is super important to me, a longer, more fragile nose on Coda did not make the cut. The world is full of instruments that require coddling, that you usually leave home because they are too fragile and/or awkward to carry. That's not what I love or what I make. I'll leave that to other folks. For instance, on recent trips to Maine, Texas, and the Berkshire mountains in Massachusetts (not to mention my frequent trail runs), I've had some really wonderful experiences playing Coda alone and for others. On all the trips mentioned above, no one else ever played an instrument because everyone else had left their instrument at home. My tiny, tough, light instrument, however, was right there, clipped to my front pocket or to my belt where it usually is.
Besides being more fragile, adding a slide to Coda would have made it much more expensive for you to buy because of the manufacturing challenges.
Finally, giving you the option of being able to conveniently clean or polish your windways from time to time was really important to me. (One of my slide mouthpieces was even detachable and re-attachable with powerful magnets so that you could remove it to access the windways: talk about complex!) A solid-state design makes accessing the windways much easier.
Ultimately, we felt that the simpler design was simply better.