Uno can have 17 different SPI devices, 3 pins go to each device- SCK, MOSI, MISO - leaving up to 17 free for chip select/slave select. Keep 2 back for Serial port for debugging, still leaves 15.Libraries might need tweaking to support that. And if using shields, may have to do some cuts & jumpers to make sure pins are not conflicting.Why not have the master poll the slaves for data? Master: "#1, got data?"#1: "No"Master: "#x, got data?"#x: "Yes, here it is"Master: "#x, data received".Then Master sends out via ethernet as needed. Can also be used to ensure each field device is still active.
On one of the shields, you will have to cut a trace from pin 10 to where-ever it connects and connect say pin 9 to that where-ever location instead.As far as the software, that's not really my area of expertise, I have not used the software for either shield and don't really know what is needed to change the SS used.