How many cables come into the j-box? If there are two or more, then you almost certainly have a neutral wire to work with. If there is only one, then the switch is downstream of the light that it controlls, but you may still have a ground wire available.
For inline wiring without a neutral/ground, you will have to use very little current in the "off" state so that the light will stay cold. Do not assume that the fixture is safe to mess with in this condition- it is still active, just not glowing.
Either way you will need a switching device, and whether mecanical or solid state you will need to be certain that it will not heat up excessively. I believe the heat is one of the contributing factors to dimmers only being available up to 5A. With that in mind, until you find out from someone who is an authority on the subject, you should assume that 5W is the maximum that can be radiated from a standard junction box.
That being said, since the controller will need to be powered when the light is on, the load depends heavily on the voltage requirements of the controller.
For example: I will assume that the "off" current will be supplied by a capacitor in series with the load to reactively limit the current. The controller is connected to an optimized filter/regulator and a bridge rectifier. Total losses through the recitfier and regulator are 2.5v and the controller needs 3V. That's 5.5V, and it won't change when the load is turned on. Five watts over 5.5V, that's 0.9A.
What's more, you'll have to find a way of using 900mA at 3V when the load is on, or your regulation will get more complicated.
Far from impossible, just sticky. I hope you can shoehorn it in the box.
<edited for spelling>
