[exim] deliver to other folder than quota root?

Hello,

in an appendfile transport there is always a Maildir to which the email
will be delivered. This Maildir will be used to calculate the quota. How
can I use this base-Maildir (the INBOX) as quota root while still
delivering to another folder (a subfolder of the INBOX)?

I could specify the subfolder as the base Maildir, but then exim would
calculate the quota only from within this subfolder, no counting all the
files within the base Maildir besides the subfolder.

Regards
Marten