Are there any APM SAML related iRule events/commands?

If I may add, what you're essentially trying to do is modify the response from the APM agent, and this would require a response event in the client side context (responses from the BIG-IP itself). Unfortunately this doesn't exist. The HTTP response events work in the server side context and are triggered when the backend server responds to a request. The easiest option, in my opinion, is a layered VIP in front of the SP VIP. The only complexity this should really add is SSL offload.

when HTTP_RESPONSE {
    if { ...look for the SAML response ... } {
        HTTP::collect [HTTP::header Content-Length]
    }
}
when HTTP_RESPONSE_DATA {
    ... look for and replace URI content in the JavaScript POST payload ...
}
when CLIENT_ACCEPTED {
    virtual apm-vs
}