How good are you with old dos-style batch files? I think you could do it with those, however I'm a bit rusty on that regard, so I hoped someone else would answer that.
Basically you'd need to extract your messages to a temp directory, copy the files to their new name and delete the files with the original names. That way you can delete any message.html files before the renmaing/copying so you don't store any html message bodies.
You can start your batch file with the 'run external command' action and that accepts %OSubj as parameter.
A more specific answer would cost me too much time (un-rusting my ancient ms-dos skills) and I'm rather short on time at the moment.