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?
This automatic rebase is a necessary operation to keep the feature branch usable after reintegration (see Keeping a Reintegrated Branch Alive).
In order to skip this rebase, the job for the feature branch should be disabled or deleted. Otherwise, the next rebase on the feature branch will likelly generate spurious conflicts.
In my opinion, if you don't want the automatic rebasing after reintegration you don't want to use the feature branch anymore. Am I correct ?