[Exim] System quotas

I've implemented e-mail quotas as part of the general disk usage quotas
in the operating system (Linux), mainly so they'll be consistent between
exim and IMAP. I set up a quota_warning at 75% of the default quota.
But if I change someone's quota in the OS, exim has no way to determine
this and warns them when they hit 75% of the default quota rather than
75% of their own quota. I was thinking of patching exim to read the
user's quota from the OS. Has anyone tried this before? Suggestions?
Caveats?
Thanks,
John Dalbec