mmm, interesting Bruce. I never check the datasheet for that because i've never never never use USART interrupt to wake-up an application... great to know that!!

SO, The choice you have is to use the interrupt on PORTB change. In the interrupt handler, check if it's an RB0 or other. Link another PORTB pin to the USART RX and that's it... in this case it will be safer to send one or few "header character" to ensure getting all data from your PC.

There's maybe some other way to do that but this is the only one who comes here... can also be done with some FlowControl pin on the DB9...