P5 lacks a method to just turn lights on and off without affecting the Intensity (SetLightOn(x) was introduced in P6).

I suspect that the only way to achieve what you want will be to write a pose file for the current light intensities, so that you can later restore them.
This is more complicated than what would be needed for P6 and above, so you may want to look at ockham's LightPanel script to see how that sort of thing is done.