I am interested in clustering small SBC into the smallest form factor.

Nanopi boards seem to be great option for this as the phy signals come out and can connect transformerless to a suitable switch IC (or perhaps router) with phy signals. The only downside is power is on opposite side to phy so you can't stand board on edge with minimal pins.

I am going to experiment with a couple of nanopi duo.

But in the meantime has anyone else tried transformerless ethernet straight between sbc and router chips and has some experiences to share?

PS, look at gumstix stagecoach concept for example.