The recent fix for issue #3895 ("repos layer does not verify mergeinfo syntax") implies that the syntax of svn:mergeinfo is frozen and cannot be changed in future release without breaking compatibility of newer clients with 1.7 servers.
We also backported this change to 1.6.17. I am pointing this out on dev@ since it might not be obvious to those who haven't reviewed the change. Do people think that this might cause problems? I would say it is not a problem because current clients will already error out hard if the syntax is changed (see also issue #3896) so we're effectively already stuck with the current syntax. We can always add a new property (e.g. svn:mergeinfo2) if we want to add information or use a new syntax. Thoughts?