The consensus seemed to be [1] that we can add/remove/change private APIs during the 1.7.x series, because the set of libraries should always be upgraded/downgraded as a set, not individually.
Therefore I intend to mark the item as "Deferred" and coloured green in <http://subversion.apache.org/roadmap.html>. - Julian [1] Round about this message in the "1.7 Roadmap Items Evaluation" thread: <http://svn.haxx.se/dev/archive-2011-04/0193.shtml>.