Did you consider status codes 502 and 504?
502 – The server while acting as a gateway or a proxy,
received an invalid response from the upstream server it accessed
in attempting to fulfill the request.
504 – The server, while acting as a gateway or proxy,
did not receive a timely response from the upstream server
specified by the URI (e.g. HTTP, FTP, LDAP)
or some other auxiliary server (e.g. DNS) it needed to access
in attempting to complete the request.
Of course, this would require a broad interpretation of “gateway” (implementation of interface A requiring a call to interface B), applied to the application layer. But this could be a nice way to say : “I cannot answer but it’s not my fault nor yours”.