Is the nRF24L01 IRQ line open-collector?
If however, the nRF24L01 has a full push/pull driver on the IRQ line,
  then this is impossible, and instead I'll have to buffer it somehow
  (perhaps with a 7407 or similar).


If it's push/pull then diode OR the two outputs with a single pull-up resistor - this would save a buffer chip.