The plugin use svn merge with the --reintegrate option to replicate the feature branch changes back into the upstream branch. This requires the feature branch to be synced with the upstream branch (http://svnbook.red-bean.com/en/1.7/svn.branchmerge.basicmerging.html#ftn.idp11155712).
This is not correct to use the plugin for a release branch.
If I understand correctly, you want to automatically merge every changes from the release branch to the trunk. As if the trunk was a feature branch of the release. You could use the plugin to do that (auto-rebasing the release to the trunk) but you will also have to make sure the trunk is never reintegrated to the release branch.
Maybe it would be simpler to add a svn merge from release branch as a pre-build step of the trunk branch?