HTTP Host Header replacement using AS3

I am using L7 policy within AS3 to manage my sites. I have a requirement where I need to modify the Host header before forwarding the request to the pool. I know this is easy in the GUI in the action section where I can just use replace HTTP Host. However, I do not see an action "replace" for the "Policy_Action_HTTP_Header" in the AS3 schema. Has anybody done this header replacement using AS3 ?
Note : I would rather not to use "tcl:.." & am looking native L7 syntax.

Any help would be greatly appreciated.