[exim] Is it possible to send custom headers to SpamAssassin…

Using the exiscan-based spam ACL stuff built in to exim these days, is it possible to feed custom headers to SpamAssassin? I'm currently testing with the exim build in Ubuntu 14.04 (4.82), and if I have something like this in the mail section:

warn
  add_header = X-Test-TLS: $tls_in_cipher
warn
  add_header = X-Test-Rcpt: $recipients
warn
  spam = spamassassin
  add_header = X-Spam-Score: $spam_score\n\
            X-Spam-Score-Int: $spam_score_int\n\
            X-Spam-Bar: $spam_bar\n\
            X-Spam-Level: ${tr{$spam_bar}{+}{*}}\n\
            X-Spam-Report: $spam_report

Both X-Test-TLS and X-Test-Rcpt will appear in the delivered message, but these two headers are not passed through to SA (which I've verified by tcpdumpping port 783). I've only been able to get it to pass the local Received, X-Envelope-From and X-Envelope-To headers, but no custom headers.

Is there any way to include custom headers that make it through to SA via the built-in ACL scanning?

--
Joaquin Lopez