Modify APM webtop favicon.ico

Interesting. So this doesn't work for you?

when CLIENT_ACCEPTED {
    ACCESS::restrict_irule_events disable 
}
when HTTP_REQUEST {
    if { [HTTP::uri] equals "/favicon.ico" } {
        HTTP::respond 200 content [ifile get my_favicon] "Content-Type" "image/x-icon"
    }
}

I testing this successfully on 11.3 HF5

Any chance the favicon you're using is unsupported?