Doing a testmerge during a pre-commit hook - svnlook detail question

Hello together,  in my company we want to start using "testing" and "release" branches. To ease work for my developers and to reduce merge times, I have the following idea in mind: Everytime a developer commits into trunc and the log message contains "@testing", I want to use that commit and automagically merge it into the "testing" branch.   I already learned that I should not do that from within a transaction, but in a post-commit hook, but I still want to give my users some feedback, so I thought about doing a dry-run in pre-commit, and if that succeeds, I do the real thing in post-commit.  I already have the testing branch checked out on my svn server, and am able to update it during pre-commit. But "svn merge" won't accept a transaction name as revision, so I am stuck here. I also don't know how to interpret the output of svnlook in a way that I can apply it like a change or patch to see if things would succeed.  Q: Is there any way to get the required information during pre-commit by using svnlook? Q: Is there any way to make svn merge access the transaction? (I believe not since it's a client command) Q: Any other idea how I could solve my problem? How do you handle different branches without having the users commit or merge to different branches?