Creating a custom error page for APM

Can you elaborate a bit more on app flow? Are you referring to APM as an IdP asserting for a set of SAML-based apps?

I think the easiest approach might be to use an iRule applied to a VIP and serve up content based on the request. Here's a sample that uses two common techniques:

when HTTP_REQUEST {
    switch -glob -- [string tolower [HTTP::uri] {
        "/error1*" {
            HTTP::respond 200 content "HTML content goes here"
        }
        "/error2*" {
            HTTP::respond 200 content [ifile get errorpage2]
        }
        "/images/errorlogo.png" {
            HTTP::respond 200 content [ifile get errorlogo] "Content-Type" "image/png"
        }
    }
}

The first approach just stuffs the HTML into the HTTP::respond, while the second and third allow you to upload an "ifile" object (HTML page, images, etc), and then reference them directly. You might apply this to a separate VIP that just handles these requests, or you could technically apply it to the APM IdP VIP (as the immediate HTTP responses would block APM from engaging).