My airline allows it. We often do flight towards Egypt, Israel where if we have more than 10 minutes of delay we are to use the unforeseen circumstances. OF course, when an airlines uses this they have to show to the CAA that not more than 30% of those flight were operated without unforeseen circumstances.
EU-OPS only gives details about normal max FDP. They say extension due to augmented crew are up to the CAA to decide.
OPS 1.1115:
Flight crew augmentation
The Authority shall set the requirements in connection with the augmentation of a basic flight crew for the purpose of
extending the flight duty period beyond the limits in OPS 1.1105 above.
With a 2 man crew the maximum we can do is 16 hours (including unforeseen circumstances).