@Rob: the time SS is low would be the time the other party is sending data. Yes, they will have to set it HIGH after sending, otherwise it would ruin the system, granted. However, seeing that it takes about 5 us overhead to run an ISR (got this number from Nick Gammon's site) I wouldn't want to enter the interrupt for every bit, since the minimum I expect is 8 bits. I will try and use some more useful names 
@David: I don't know. Doable, yes, workable, maybe. I might give it a shot. The reason I did not choose the interrupt on the clock pin was because the clock pin will oscillate as well when other spi devices are accessed, so any ISR connected to SCK will have to start with asserting that SS is LOW. However, it does make sense.
Both thank you for your input.