I transferred in CMB from CX to QR and can confirm you can use the self printed boarding pass. The transfer is very easy and convenient. Also, QR will send you to the Araliya Lounge which isn't great, but you can get into the Srilankan Serendib Lounge. They will try to tell you to go to the other lounge, but if you push them,they will let you in. It is marginally better....

Also, on the flight to Doha, if it is the A340-600, try to get row 1. The foot cubby is waaaaayyyyy bigger.