A big thanks to Paul for his assistance in getting past this problem!
For anyone else encountering this particular bump in the road here's what appeared to provide a solution. I had, on Herb's advice, removed all my libraries from Directory Manager and added just the new Solo Voices library. The library still didn't show up in VI Pro however and at this point I threw up my hands and requested support. After a bit of poking around with the software, by Paul, the library finally showed up, after another of the existing libraries was added in to the Directory Manager.
Still, it's a bit of a mystery what's actually at play here and it might have been something else that led to VI Pro, Directory Manager, and the Solo Voices library to finally play nicely together.
So, for anyone else encountering this issue, assuming all the software is up to date, try removing all libraries from Directory Manager, then add Solo Voices, then add another working library back in.
I haven't had much time to work with the Solo Voices library beyond building my standard template for each voice, but even that small encounter suggests the library has a lovely warmth and human quality to it. A couple of times I noticed a performance transition jumping out with an out of character formant shift (vowel change), and there's a pretty extreme high pitch, again out of character, in one of the the bass voice patches that I would want to avoid, but overall it sounds great and I'm looking forward to working with the level of nuance and striking realism the performance patches, in particular, provide.