I wrote a little Windows batch script, svnloghack, that makes it easier to change the revision logs in a Subversion repository. It takes a path to a subversion repository and a list of revision numbers as command line arguments. It creates a file named zsvnhackrev.txt for each revision (where rev is the revision number). Each file contains the corresponding log message. It also creates a batch script named zsvnhack.bat that writes back all of the log messages to their corresponding revisions in the repository (presumably after you have edited them).
Also, the batch script pre-revprop-change.bat is a minimalist hook script for Subversion that will permit log changes (and other revision property changes) to take place. You have to put it in the hooks directory of the repository if there isn't such a hook already there.