[Exim] maximum size allowed in headers_add

I am attempting to add an extremely long header containing a list of addresses (the expansion of the $recipients variable) to messages being saved in an internal folder for various compliance purposes. I have a system filter setup that does this without a hitch - but I have 2 questions:

1. Is there a way to do it that does not require me to add the headers with a system filter and then remove them on every outgoing router? I need to save the BCC addresses, but obviously I don't want them attached to every outgoing messages. Normally I would simply add the header using a shadow router with no_more = false, but the $recipients variable is only available in ACL's and the system_filter.

2. More importantly, some users are using very long sender lists and the system filter fails on those messages with a message about the string being too long for headers add. Does anyone know what the size limit for headers add is, and if so, is it set in the RFC's? Arbitrarily in the code? Can it be safely circumvented?

The system filter being used is:

if first_delivery then
       headers add "X-Foo-Mail-From: $sender_address\nX-Foo-Mail-To: $recipients"
       unseen save "/home/willcap/mailusers/audit/"
endif

thanks in advance,
David Powers