Hi,
The MC1321x Reference Manual describes the Energy Detect function as taking 144uS to warm-up the receiver and another 128uS to measure the received power on that channel. The Energy Detect function is completed when the cca_irq is asserted. Once you get the cca_irq interrupt, you will need to read the average power detected from the cca_final[] register, and determine if you got something. If not, you will need to do another Energy Detect function. Seems like you could perform back to back Energy Detect functions to cover the period of one second and pick up the PAN. This would not decode the frame, but tell you something was out there on a specific channel.
Thanks,
Rick