It seems like _BESRelay_GatherMirror_UpstreamCheckPeriodMinutes is the setting that would resolve the issue of a relay behind a NAT or similar. The default value seems to be 6 hours, but if you set that to something more aggressive, then that should be the about the max time to wait for new content to bridge the gap.
It would be better if port 52311 could be forwarded on the NAT to the child relay behind it.