I seem to come to a different decision each time I look at it
. As noted in post #3 above, my original version actually used "knew" - past tense, matching the "was" is both clauses. (It got changed over accidentally when I was writing the OP.) With that in mind, it is more apparent that one way to rephrase the sentence would be:
Abby was gone, and who knew where Bill was now?
(A question mark seems necessary here.) If you tried to use "knows" in this version it would be obviously wrong: "Abby was gone, and who knows where Bill was now?" It would have to become "Abby is gone, and who knows where Bill is now?" - and it wouldn't fit the context very well (self pity is mostly past tense, it seems to me).
The above version avoids the need for hyphens, but I am not thrilled with it becoming a question, it's not a question in the character's mind, it's a defeated acknowledgement that he doesn't know and can't find out.
Looking at the two variations of my original sentence next to each other:
Abby was gone, and Bill was who-knew-where now.
Abby was gone, and Bill was who knew where now.
I think I can see the real benefit in the hyphens. Without them that second clause in the second sentence could (at a stretch) be interpreted as: Bill was the one that knew where Abby had gone. The hyphens make the intention more obvious, I think. (I may think differently when I look at it again tomorrow.
)
I'm glad not every sentence gives me this much trouble. There's a 150,000 words in this second novel!