Today, we can chose to allow or forbid empty SVN commit message with the parameter sys_allow_empty_svn_commit_message. This behavior applies to all repositories (platform level). So, it is not possible to make commit message mandatory (without a reference) only for some projects/repositories.

It should be more flexible to store this property at repository level (like it's done with plugin_svn_hook_config.mandatory_reference).

Another option may be to allow to declare paths in repository where references are not mandatory (this may be useful in some specific use cases). So, to require messages allowing to not use references, administrators may select the option Force every commit to include a reference with the the path / as exception.