Thanks again for all the replies. I wish I could have come back to the thread sooner.
In my case GallopingGames' solution doesn't apply, since the ePub in question already had separate html files: cover, toc, title-page, ch1...
When I mark the title page as the "text" element in the guide, the file opens to the middle of the toc. Mark ch2, and it opens somewhere in ch1. Ch12, and it opens in ch10, and so on, with the gap widening to as much as 10 chapters (there are 70-odd chapters in the book).
The only thing that works is putting the toc at the end in the spine, so the files are ordered: cover, title-page, ... where title-page is the desired start point. But that's not what the client wanted.