Hi Tom
It is encouraging that the basic configuration does work. I myself am interested in MQTT communication over ethernet.
Compiling my Sketch with RFM69 Ethernet and the MQTT PubSubClient client, indicated 68% Flash consumed, and 66% SRAM consumed, this is with other stuff as well so there is some headroom
My investigation has shown that 1.5.8 has the new SPI libraries included, but also that the W5100 ethernet drivers have also been enhanced to use these features.
I am thinking that IF the new SPI library transaction support could be incorporated into the RFM69 library, then this might be the best outcome to deliver a stable solution.
Kiwi