Reselect pool on status code 500 or above...

That should work. The only change I can see you should make is to limit retries to GET requests. Else, you'd need to collect the HTTP payload for POSTs and append it to the request headers and then call that combined variable with HTTP::retry. That could eat up a lot of memory.

You can set a variable in HTTP_REQUEST if HTTP::method eq "GET" and then check that in your if statement looking for a 5xx status:

when HTTP_REQUEST {

   if {[HTTP::method] eq "GET"}{
      set request_headers [HTTP::request]
      set method "GET"
   } else {
      set method ""
   }
}
...
when HTTP_RESPONSE {
   if { [HTTP::status] starts_with "5" and $method eq "GET"} {

   ...
}

Also, if the retries exceeds the number of pool members, you could send back a generic HTTP 200.

Aaron