Can you "su -" in MySQL?
I just realized -- so long as you don't mind locking out the user while you log in --


back up the mysql.user table (well, the user's hashed password, at the very least)
set their password to something you know : UPDATE mysql.user SET password=PASSWORD('new password') WHERE user='username' AND host='hostname';
log in as them
set their password back to what it was : UPDATE mysql.user SET password='saved password hash' WHERE user='username' AND host='hostname';


... you may need to flush privileges; after manipulating the mysql.user table.