I have just had a quick look at rules and I am not sure how it can be done. Rules still use the concept of NEW and OLD. If my original row has 'myfield' set to 'me' then I don't think I can tell the difference between:
Update mytable set afield='something'
and
Update mytable set afield='something',myfield='me'
Within the rule I think NEW.myfield will be set to 'me' in both cases. Please can you explain how I can tell the difference between the two update statements