HTTP_request and HTTP_response in same irule?

the plan is to divert traffic to a specific node based on a header value in request, but for calls that don't have a header value, the big ip should round robin the connection and then add a header value to it during response based on the responding node.

Just keep in mind that headers are generally not relayed by the client. In other wordsm if you send a header to a client browser, it will not send it back.