Do you want to be able to add a maximum power limit switch, set at 250W?
If so, and if the Crystalyte controller is, in reality, a Xiechang controller sold by Crystalyte (some are, some aren't) then you can get pretty close to this by using the speed switch capability. This is fairly straightforward, although may need opening up the controller, adding some wires and also re-programming the controller.
This does depend on the exact model of controller you have, though, and to determine that you may need to open it up and have look, maybe taking some photos so we can see what you've got.
The way to do this, if the controller does turn out to be a Xiechang, is to wire up one of the speed switch settings and then programme it to a low % speed. A simple low current switch, maybe hidden if you want it as a "legal mode" switch, could then be used.
This won't be a true power restricter, it will be a maximum speed restricter, but the effect might be good enough for your needs.