So, I'm mainly posting this as an FYI. I'm going to contact my dealer about this tomorrow morning, but thought I'd go ahead and post about what I've found. I'll see how much better it rides after this fix...
I was noticing a lot of resistance in the chain line, and was looking at idler solutions to fix it. In the mean time, I figured I'd go ahead and isolate the problem to the idlers, to ensure it wasn't anything else. (I may still go for an idler upgrade, but that wasn't the big problem.)
Here's the TerraTrike Dealer Assembly Manual from 2008, you can find it in the manuals section: http://www.terratrike.com/media/docs/Te ... al2008.pdf
Also, the video on YouTube, showing proper chain routing: http://www.youtube.com/watch?v=CMUD7r5GIKs
There's a problem, though... on some Paths, that is incorrect
. (I don't care what the manual says.) It appears that early Paths, when that manual was written, used vertical dropouts, and a chain tensioner to take up slack in the chain. This put the return end of the chain well below the idler, and is the correct chain routing for that layout. However, my 2011 Path 8 uses track ends, and no chain tensioner. The chain routing that TerraTrike recommends is completely inappropriate for the Path! On an 8-speed Path, which has a 25T rear sprocket, the chain floats slightly above the idler, but rubs gently on the frame. On a 3-speed Path, with a 22T rear sprocket, it'd be much worse - the chain would be forced to bend around the frame.
I've relocated the return chain to BELOW the outer rear idler, contrary to the manual. Now, the chain is well clear of the frame.
As a side note, what's the best match for touch-up paint for the Path? I've got some damage to the paint on my frame, some of which was inflicted by this chain issue (although that'll require some rust cleanup it seems, and like I said, I'll talk to my dealer about that), some of which was inflicted by my attempts to lock the trike when I started commuting with it.
Here, have some pictures.
This shows my current chain routing, with the red line being how the chain routed before I modified the routing.
Same photo, just with the red line removed, and a red circle around the damage caused by the chain.