capturing and appending to HTTP::payload in a POST to a SAML assertions consumer URL

The layered LTM VIP would have to sit directly in front of the IdP or SP (VIP?) and thus should not require any modification to URLs. So for example, let's say you're the IdP. The SP sends an assertion request to the IdP. That traffic passes through the layered LTM VIP and the response with the SAMLResponse payload can be edited to include this new header (on its way to the client). Now let's say you're the SP. The SP traffic flows through the layered LTM VIP, so any POST request passing through it with the SAMLResponse payload can be edited to include this new header.