I have seen another method that is less accurate, but it could work. It consists in a black heatsink attached to a diode with a metal package (i.e. TO-220). The diode is used as temperature sensor, and measuring ambient and heatsink temperatures will allow you to calculate the power of the laser, which is pointing to the heatsink generating a rise of temperature. You will have to know the total thermal resistance to achieve good results, which can be obtained with callibration.
It's another method, maybe it's useless to you since you must use a phototransistor, but maybe it's useful to somebody else.