Rate limiting by auth username in rcpt to

Using metermaid,throttle, I have configured FROM_ACCESS mappings to rate limit the number of mail froms a given SMTP AUTH username can insert. However when I go to apply a similar rate limit in SEND_ACCESS I can only store the mail from/env from value. We are an ISP with some hosted domains so there is a valid use case for SMTP AUTH where the mail from and auth username values will be different, ergo I cannot use authrewrite to rewrite the user provided mail from to the auth username used.
What are my options for storing auth username when counting rcpt tos?