@naoki yes my setup is connected on the DC side.
A small switching regulator do a 34V to 12V conversion and that this 12V that is used to control CPU fans. It's also used by the linear reg to supply 3.3V to the arduino, radio and sensors.
From what I read it is totally fine to play on the DC side of recent CC drivers. These are technically voltage and current limited, so they won't try to supply infinite voltage (LPC-60 is limited to 34V, which is what I want to get 60W). Old analog CC should not be used.
You also need good MOSFETs to be sure to have a clean regulation.
Then, it may be true that the driver don't like it I don't have enough background in electronics to be sure. Only thing I can tell you is that other people did it before me (sadly I did not invent anything) and it's been running for about 6 months (4 months on breadboard before getting to the custom PCB point) without any trouble, no overheating of the driver or anything. Also these LEDs driver are only $14 so even if they die in 5 years it would not be a big problem.
As @pmatil say, it may be cleaner to cut AC but I don't think you could dim the light this way (~500ms for current rise of the driver) but if you just want to turn something on/off it would be easier, and you can directly use a commercial solution. If you want to do it yourself with arduino, I would recommend you to look for a good SSR (solid state relay), plenty of options on the mysensors forum too for that!