Did this just happen once ? Or is this a chronic issue ? Maybe I have some type of denial personality, but I would change the coolant and change the oil to see if this happens again. Due to the location of the radiators and coolant, it seems more likely the coolant could get into the oil and seems unlikely that oil would get into the coolant. But then again anything is possible. I fail to see how a head gasket can allow coolant into the transmission.
Please keep us posted, as I like a good mystery that has a happy ending.
John