A developer of ours tends not to specify messages when committing his code, I know I could jump up and down and force him to do so from here on in, but how do I deal with the previous commits.
I'd very much like to go in and edit the messages so that they are descriptive of what they do.
How might I go about doing this?
Does this SO question answer you enough? Your question seems slightly different in one nuance: that question is more for 1 commit, this is for a large chunk of commits presumably. So, for the amount of work it may take this may not be good enough, but if it's not a huge number of changes maybe this is okay.
Play around with this
$ svn propset --revprop -r 25 svn:log "Journaled about trip to New York."
property 'svn:log' set on repository revision '25'
From the Subverion site: http://svnbook.red-bean.com/en/1.4/svn.ref.svn.c.propset.html
Are you using a specific client for subversion, or just the command line? With TortoiseSVN when you have the list of log messages open, you can right-click on one and choose "Edit log message".
This link describes how to do it if you don't have TortoiseSVN (doesn't sound easy): http://weblogs.asp.net/psteele/archive/2008/01/24/subversion-changing-log-messages.aspx