If a PWM PDC DMA is used, the "clock pin" frequency will be 84 MHz/8/2 = 5250000 Hz. If a 5% error is still acceptable, that's OK.
Edit: I took some more time to explore a solution since a 5 MHz interrupt frequency is an issue.
1/ It's possible to output a 4.94 quasi square wave on a "clock pin", say PWM channel 1 PWMH1
2/ As MartinL suggested, use PWM channel 0 PWMH0 to output 26 signals on a "data pin" at a 4.94 MHz frequency by using this channel as a synchro channel but only with itself (only one interruption after 26 ouput signals), and enable PWM channel 0 and channel 1 at the same time to be synchro.
3/ Once the 26 signals have been sent by PWM channel 1, disable this channel inside the interrupt handler, but keep PWM channel 0 enabled.
I guess after that you will have to decipher a receiving message from the device and attachinterrupt() won't be fast enough to decipher 5 MHz input signals....