The SPI library has MISO as input. That shouldn't affect you too much. If you want it as an output you are out of luck. According to the datasheet, page 168:
When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden according to Table 18-1 on page 168. For more details on automatic port overrides, refer to ”Alternate Port Functions” on page 80.
If forces MISO to be an input. This is nothing to do with interrupts.