nor an integrated network switch (so you don't have to run a separate 100BT network cable to each Pi). Any thoughts on this?
An effective integrated switch design requires that the MDIO signals normally fed to the Ethernet PHY be exposed (brought out to the 40 pin connector), thus allowing for the omission of those analog components. The Pi3 does not expose its MDIO signals so the only wired interface available is Ethernet on RG45. Adding a switch to this board would mean adding all of the components of an external 4 port 100baseT switch, including the 4 RJ45 connectors.You'd still need 4 Ethernet cables!