Ethernet backplane components
Perhaps someone will come along with a better 'answer' later.
But the conclusion here is pretty much just &quot;it'll be fine&quot;.
In the Ethernet PHY world, there are two different methods of driving the differential pairs, current-mode and voltage-mode.
If using current-mode, then coupling transformers will always be required.
If using voltage-mode, then for the shorter distances expected in intra-board routing the magnetics can be left aside, and just capacitors used to separate the Tx / Rx bias voltages.
So if going for an ethernet driven backplane, voltage-mode PHYs should be selected, with switch architecture then 'as desired' for the application (i.e. either a star network with a large port count central switch, or a linear network with numerous low port count distributed switches).  Latency of the switching elements would need to be factored in, along with overall power usage.
Trace impedance should be matched to the 100ohms, with Tx/Rx termination resistors as required (if not internally terminated already) and coupling capacitors of ~0.47uF are a good enough start.  The coupling capacitor value may require some tweaking due to transceiver characteristics (different vendor / series).
http://ww1.microchip.com/downloads/jp/AppNotes/jp578072.pdf
http://ww1.microchip.com/downloads/cn/AppNotes/cn586761.pdf