Need to rewrite a 500 status to 200, but how ?

I don't see why that wouldn't work, but disabling chunking on the request might interfere with the application ...

# iRule to convert 500s to 200s
when HTTP_REQUEST {
  # Don't allow data to be chunked
  if { [HTTP::version] eq "1.1" } {
    if { [HTTP::header is_keepalive] } {
      HTTP::header replace "Connection" "Keep-Alive"
    }
     HTTP::version "1.0"
  }
}
when HTTP_RESPONSE {
   # grab response of a 500
   if {[HTTP::status] == 500}{
      HTTP::collect [HTTP::header Content-Length]
   }
}
 
when HTTP_RESPONSE_DATA {
   # change response to 200 and send
   HTTP::respond 200 content [HTTP::payload] "Content-Length" [HTTP::header Content-Length] "Content-Type" "text/html"
}

Just a comment on the wisdom of replacing a status 500 with a status 200 - I know it's your your web application, but application frameworks use a specific response for a reason - if you get a 500 error response, it should only be because the server context is now broken and nothing further should be attempted. It's not like a 401 Authentication Required where the application can proceed with additional credentials.

And leaking information like that (username does not exist) is just asking to be abused by an attacker. I'd think very carefully before making that sort of change.