A few changes to this one tonight:
The muffler system might be solved, but I couldn't get enough testing to know for sure before it started raining again. With the copper inserted into the small inner sleeve that doubles as the exhaust tip, I had just enough space to drill two small mounting holes for short bolts to hold it in place. Then, I just had to put some stovepipe around the outside edges with a hose clamp to close the area the inner pipe didn't quite fill. Once the makeshift fitting was assembled, the heater hose easily attached to the copper end remaining with another hose clamp. There was a couple feet of it, so I wrapped it down and around the engine/tire and secured it near the end and bottom points. A long spring wrapped around the frame/motor mount above the cassette secures the bottom, and a zip tie holds the end near the tail light at a 90 degree angle off the side. It's performed well so far, but still needs a couple rides to be certain it will hold.
This little exhaust experiment led me to another issue that was much more potentially dangerous. As I attached the new exhaust hose, I happened to hit the back tire against my arm. This made the wheel shake back and forth with a vengeance, and told me the hub bearings were loose. I quickly removed the back tire and secured it tightly, as if my life depends on it (as it does). Nothing was stripped out yet, so I'm glad I caught it early. It seemed to be well seated before, but I imagine the speeds it produces requires more torque on the nuts. No more play in it now, but I'll be checking that wheel often.
After bringing it in from the rain, I noticed a loud, sickly wheezing coming from the engine. Typical problem: No gas cap vent. Don't know how that one slipped by the goalie. The smallest bit I could find was 1/16", but it worked. After drilling the hole, the wheeze is gone. No spillage either, only a small circle of condensation around the hole.