Simply get the transform.eulerAngles.y (or x or z whatever you want) and put it in a float variable and then serialize it. when sending there is no problem. when receiving you should change the y rotation of the object to what you want and keep x and z with their previews values. the code would be something like this (the pother parts can be your own code).

float yRot;
void OnSerializeNetworkView (BitStream stream)
{
if (stream.isWriting)
{
float rot;
rot = yRot;
stream.Serialize (ref rot); //sending y axis rotation.
}
else
{
stream.Serialize (ref rot);
yRot = rot;
transform.rotation = Quaternion.Euler (transform.eulerAngles.x,yRot,transform.eulerAngles.z);
}
}

using quaternions is not easy so don't use them if you really don't know them the Quaternion.Euler method takes the rotation as x,y,z in eulerAngles (360 degree angles that we know well) and then returns a quaternion value that is the same rotation and you can set transform.rotation to it. i did not tested but transform.eulerAngles might be writable too.