As of SGOS 6.5.9.2 it is now possible to set ICAP request headers for both REQMOD and RESPMOD but only via CPL
This feature is not yet available in SGOS 6.6.x but is expected to be available in a later version.
For details on how to use this feature please refer to the latest version of the CLP guide fir SGOS 6.5 https://support.symantec.com/en_US/article.DOC10097.html