QR seems to have disallowed that construction. They file no fares on DUB-BAK (BAK being the city code including GYD). Further, as is their standard policy, their fares are not combinable; i.e., you cannot take DUB-DOH and add it to DOH-GYD.

It may be an oversight, as they are happy to allow LON-GYD.

BA.com doesn't recognize a lot of Avios destinations, so you could call. But as I understand, the Avios tax+fee calculation is done by taking a revenue fare calculation and removing the fare from the calculation. So it may not work either.