Apache httpd workers retry
I have spoken to a Dev on httpd mailing list and he confirmed that this feature has not been implemented in the current code base. I have also reviewed the code myself. The state machine inside mod_proxy_balancer cannot retry on HTTP error but can only do so in case of connection error. I tried to implement it myself but its too complicated and I can easily make the code base unstable.
So this problem has no known solution today...