Wanted to pop back into this thread to report that everything's settled and I've found 5 string fifths Moksha on the Shen 5'r with the following strings:
C - Spirocore Mittel LOW B tuned up 1/2 step
G - Spirocore Red Mitchell 3/4 mittel
D - Thomastic Dominant D
A - 40 gauge Damian Dlugolecki gut
E - 27 gauge Damian Dlugolecki gut
I switched out the pickup for a Yamahiko single element bass side, put in the capacitor mod (which brought the pup to life, IMHO), but had to add a couple of thin Koa wood shims above the adjusters after everything settled to get the 'base line' straight height right. I was planning on recutting the bridge with a premium ($$$) 4/4 one, bought the blank, but am going to hold off because this one is speaking just fine.
For some reason, it seems like the D string on this axe really sets the tone for the whole instrument. I tried everything I could - all the Spirocores, plain gut, gamut pistoy, Oliv, etc. The Dominant D really was the special sauce that bridged the whole instrument.
My boy Garry Goodman at octave4plus.com hasn't given up on a steel wound Spirocore-like flat wound high E. He's working on the 3rd prototype right now and must have the patience of a saint. He says the trick to getting really thin bass strings to hold tension is to tune crazy slowly - like over the course of several weeks, 1/4 turn every couple hours. I f'd up the previous two by trying to crank them up to tension too fast, but he says once they're stable, if you're patient enough, they should last 5-10 years.
The plain gut is responding marvelously under the bow, but I am curious to see if we can make an all steel set work.