Overall view of new chain routing. Compare to this earlier version.
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After posting a message to alt.rec.bicycles.recumbent usenet newsgroup, Michael T. Flaman responded with this:
"Also, when the rear wheel goes over a road bump, it would seem that the lower section of the chain is required to significantly lengthen. Is this any problem regarding the ability of the rear derailleur to accommodate this situation - as well as accommodating the front/rear gear changes?"
I really hadn't thought about this, but it seems this would be a problem. I poked around the pivot point a bit and realized there was barely enough room to snake the chain through the rear most triangle rather than below it, if I switched to a narrower tire and removed the canteliver brake posts. I did, and drilled a hole in a tube to braze a nut in. And now the idler wheel bolts in as seen below.
Overall view of new chain routing. Compare to this earlier version.
A close up showing the places on the rear triangle where the rear cantilever brake posts were removed, the vintage Shimano Dura Ace sidepull caliper brake, the new swoopy brake cable routing and the new position of the rear idler pulley. The old pulley's metal cage is gone too, the one that took a whole evening to make and attach. Sigh. This definately took care of any chain being used up in the flexing of the suspension though.
An underside view showing the brake caliper.
The pulley. I used a hole saw to cut out the lexan disks for chain guides. Lexan's pretty tough, I use it for water rocket fins. We'll see how it does here. Looks kind of high tech though, and it is light.