Hello Phogi,
supposing cam is the first camera in your chunk, then to assign ( Y, P, R) as reference orientation angles (yaw, pitch, roll) you can use
cam.reference.rotation = PhotoScan.Vector( [Y, P, R] )
as in following console extract:
In [1]: chunk = PhotoScan.app.document.chunk
In [2]: cam = chunk.cameras[0]
In [3]: cam.reference.rotation
Out[3]: 2018-11-26 01:42:56 Vector([312.9, 0.0, -0.0])
In [4]: cam.reference.rotation = PhotoScan.Vector( [320, 2.0, -0.0] )
In [5]: cam.reference.rotation
Out[5]: 2018-11-26 01:42:56 Vector([320, 2.0, -0.0])