PWMing the output of a constant current driver
It really depends on how the constant current driver has been designed.  


If you place the MOSFET in series with the LED, the constant current driver will most likely raise its output voltage until it hits its voltage limit. If the driver has no overvoltage protection (very unlikely) it will fry itself. Every time the transistor turns back on the LED will be exposed to momentary overvoltage as the driver feedback readjusts itself, which might damage the LED in some cases.  








simulate this circuit &ndash; Schematic created using CircuitLab


If you place the MOSFET in parallel with the LED the constant current driver will most likely lower its output voltage close to zero in order to maintain the current at the setpoint. Again, some drivers might behave unpredictably if they are designed with a minimum output voltage in mind. 
The LED however will not be exposed to overvoltage in this case, as the driver will have to ramp the voltage up, not down.  








simulate this circuit

In any case the driver will most likely not appreciate the wildly fluctuating load, the best option would be to just control the setpoint of the driver.