Is FFC suitable for remote Ethernet jack?
A simple cheap, easy and elegant solution would be to put the jack on the board next to the phy and run an ethernet extension to the outside of the product enclosure. There are even extensions available with built in holes for fasteners for easy installation and manufacturing. 

You might be able to get a double sided flat flex cable, but flat flex cables are not cheap unless you are buying them with quantity. And then you have to have another board at the output and control the impedance all the way from your board to the other mini PCB. 

Same goes for USB, is easier to put the connector on the board and then run an extension cable then roll your own.