So, could this mean that one could theoretically wire a MicroSD card directly into ethernet plug and with some voodoo harness PoE to create an ethernet plug with busybox on it?
No. For PoE you need a PoE controller and then you need a MAC and PHY to convert the ethernet twisted pair signals into something the microcontroller can read (PHY decodes the signal for the MAC which handles packets and etc)
You can theoretically use something like [1] or [2] to connect the SPI bus to an ethernet controller.
