You can enable Subversion log editing by installing a pre-revprop-change hook. Here is a minimal Windows batch file that will do the trick (converted from the template UNIX shell script that comes with SVN):
@echo off
setlocal
set REPOS=%1
set REV=%2
set USER=%3
set PROPNAME=%4
set ACTION=%5
if "%ACTION%" == "M" if "%PROPNAME%" == "svn:log" exit 0
echo Changing revision properties other than svn:log is prohibited >&2
exit 1
