When using postfix, prepending the X-Envelope-To header will reveal all BCC recipients.
I might not have found the right place for prepending the header yet (smtpd_recipient_restrictions). Please correct me if there is another way!
As an alternative one could use recipient_bcc_maps and allow sub-addressing to take care of the original envelope recipient (which would be part of the sub-address).
E.g. my route to pile is firstname.lastname@example.org, sub-addressing of the original recipient email@example.com would be firstname.lastname@example.org.
For this to work the piler parser needs to be aware of the type of sub-addressing used.