I've been looking at this for my own business needs too - and there's a few things you've got to factor in:
1) what's to stop a staff member from permanently closing a ticket just because they can't be bothered to deal with the customer any more (or the problem is too complex for them to deal with etc)
2) the customer's got to be informed differently - so that they know that they can't reply to an email or log in to re-open it
3) unless used properly, this could seriously annoy the customer - they've asked a question, which has been misinterpreted and therefore answered insufficiently, and "permanently" closed. They can't email back to say "No, what I meant was (blah, blah, blah).."
To factor for at least 1 and 3, my thoughts go towards an "authoriser" user or manager who would need to affirm the request to permanently close the ticket. Maybe by using the department manager, or a "can authorise permanant lock" flag in the DB/staff record for example.
I will be looking at doing some work on this mod soon - unless someone gets there first, that is.... ;)
