Most of the information stored in a SVN commit can be edited. This is the case of the message and date: edition of these values just require a simple hook.
Adding the pre-revprop-change hook to allow SVN properties edition
In your repository, create a file hooks/pre-revprop-change or simply copy file hooks/pre-revprop-change.tmpl. Then ensure read and execute rights are granted for everyone. This script will be executed whenever a property is changed. The script return value tells if a modification is allowed or not. Edit the file so it looks like:
if [ "$ACTION" = "M" -a "$PROPNAME" = "svn:log" ]; then exit 0; fi
if [ "$ACTION" = "M" -a "$PROPNAME" = "svn:date" ]; then exit 0; fi
echo "Only svn:log and svn:date revision properties can be changed" >&2
Caution! This script allows anyone who has write access to the repository to change date and log. You may want to restrict this access by adding some check to the $USER value
Editing a SVN commit log or date
From your workcopy, you can edit a SVN commit log with:
~$ svn propset --revprop -r 25 svn:log "Bugfix 007."
To edit a date, just check the date format first:
~$ svn propget svn:date --revprop -r1
Then edit the svn:date property to change the date:
~$ svn propset svn:date --revprop -r1 "2007-04-18T22:00:19.308764Z"