custom error page based on /URI

Without rebuilding your entire iRule, the logic for such a requirements might look like this:

when HTTP_REQUEST {
    if { [string tolower [HTTP::uri]] starts_with "/uri" } {        
        set noerror 1
    }
}    
when HTTP_RESPONSE {    
    if { ( ( [HTTP::status] starts_with "4" ) or ( [HTTP::status] starts_with "5" ) ) and not ( [info exists noerror] ) } {        
         HTTP::respond 200 content "..."   
    }
}    

This will set a variable in the HTTP_REQUEST event if a specific URI is requested, which will be visible in the corresponding HTTP_RESPONSE event. If users might flip between applications in a single connection, you can optionally add the following to the bottom of the HTTP_RESPONSE event to clear the variable between requests if it exists:

if { [info exists noerror] } { unset noerror }