I just had a look at plugging this board into the Spark.IO using UART rather than SPI, and what I found was that this is possible using an MFRC522 based circuit, it most likely is not possible on the RFID-RC522.
Basically Pin 1(I2C) is connected directly to GND, and Pin 32 (EA) is connected directly to 3.3V. These are the two pins that select between UART/SPI/I2C mode. As set, they force the device into API mode, and I have not yet found a way to override them. In fact, I am not quite sure why there is an SDA pin on the PCB, since I have not found a way to select I2C mode.
If there was a way change mode, the MISO pin could be used as SCL in I2C mode. MOSI and SCK would be address pins. In UART mode, RX and TX would be SDA and MISO with MOSI and SCK used for handshaking
To summarise, unfortunately, with this PCB, it looks like you are stuck with SPI. Of course there might be an SPI way to change the device into UART or I2C but I could not find it looking at the data sheed.