Not standard, but for < 8mm thick, if it needs to be decently robust, I'd look at a Lemo connectors.
www.lemo.com/en/standard-range/b-connector-0 If youre only using 10/100, you only need a 4 pin connector. The EPG.00.304.HLN is only 7mm above the PCB surface, and the OD of the mating connector (FGG.00.304 ) is 6.4mm. However you gotta find a cable that <5.5mm to fit in the cord grip on the connector.
However you could get cheaty and feed only the 2 pairs of standard cat 5e into the connector, use one of the collets that have a lip for a strain relief boot, and some adhesive heat shrink to secure the cable jacket. Use FEP insulated cat5 if you dont hate the people assembling the cable harnesses, PVC insulation will pull back when soldered, really, really bad.