Svn pre-commit hook to disallow svn:mergeinfo on non-root directories

I would like to use a pre-commit hook that prevents developers from setting svn:mergeinfo on non-root directories. That is, I want to enforce that svn:mergeinfo can only be set on directories like "trunk" or "branches/branchName". Developers sometimes need to be "reminded" that it's not good practice to use a subdirectory of the root as a merge target (per the best practices listed here). Does anyone have such a hook script or know where I could find one? I am in a windows environment, so batch or powershell would be preferable, but anything would certainly be helpful.