Hi all,
just for reference:
we had a similar problem using F5's BIG-IP Web Application Firewall as reverse proxy. We did not get this working (not sure if SAP's or F5's fault) and opted for some rather peculiar (nevertheless working) solution outlined here X.509-Based Logon to NetWeaver AS from SAP Web Dispatcher - SAP Web Dispatcher - SAP Library
Maybe this helps someone looking for a solution.
Cheers
Jens