Thanks Eliot! Excellent summary of the problem and the situation.
Your method #2 (adding a map-level @keyscope) is what I tried, but it doesn't seem to work in Oxygen. It doesn't form the cross-book links properly, and if I create them by hand, they don't resolve.
I will try method #1 tomorrow. However, I'm not sure how well it would work in practice, as the writers want to share entire chapters between books, with a variety of cross-references to both book within the chapters. Trying to create all the local/cross-book flavors of indirection links would be quite maddening!
The good news is, your reply and suggestion for method #2 gives me the confidence that this should be fixable in Oxygen. Thank you so much!