Hi anj.
You can use any of the signals for RS 233 like Tx.
You can connect trhat to your INT 0 line as well as the normal serial connection that you are using.
The micro is normally at sleep state,inorder to wake it up send any rubbish serial data this will wakeup the micro.
Then pause & send the actual code you need to communicate with.
Regards.
Suded.