For what it's worth, the "answer" I was looking for was found in the last paragraph here, which tells me that Subversion will be of no help in resolving my particular problem. Joy!
There are other cases which are labelled as tree conflicts simply because the conflict involves a folder rather than a file. For example if you add a folder with the same name to both trunk and branch and then try to merge you will get a tree conflict. If you want to keep the folder from the merge target, just mark the conflict as resolved. If you want to use the one in the merge source then you need to SVN delete the one in the target first and run the merge again. If you need anything more complicated then you have to resolve manually.Other tree conflicts