It's a modification of Horde and Plesk that would be required. Both SWSoft is perfectly capable of doing. (As shown by the text ads)
It is not however a trivial modification.
You would need add way for webmail to contact the Plesk CP server to find out what the usage/limit was, then of course write something for webmail to query that.
While you could easily do this by opening up your server to attack, doing it securely is a much different problem.