With SleepWalking, you can have core stopped and SERCOM5 (in PD0) will wake up PD1 and DMA controller will transfer the byte, and go back to sleep. But you can't examine the byte and act on it without CPU intervention.
The best you can do is have the system be in a sleep mode with PD0 active for SERCOM, and wake up on the received byte. What exact power consumption to expect - I don't know, it depends on may factors, it may end up being 2 mA, UART has to consume something. I also have no idea about ASF.