You could do that easily with a database query. For example, the following query would set the titles of everyone in group #1 to 'whatever'.
UPDATE w3t_Users SET U_Title='whatever' WHERE U_Groups LIKE '%-1-%'
If you were doing this for several groups, you'd have to do in the correct order, since users may belong to more than one group.
Hmmmm ... there's also a column w3t_Posts.B_UTitle. The only place that seems to be used is in printthread.php. I wonder if that's a bug.