[exim] /etc/virtual/limit file

We have set a limit of 300 emails per user by using the /etc/virtual/limit
file as described in the DA knowledgebase.

This is fine except that we have some php scripts which send email to our
customers. We need to be able to specify that some users (mainly us) can
send more than 300 a day.

Is there a way to set a per user limit rather than just setting it globally?
I can't imagine we are the first to need this solution.