"The light fixture only needs to be a figure so it can receive the LightOn and LightOff poses..."

Anything parented to a figure, or in a parenting chain where a figure is part of the chain should take a pose, just so long as any part of the figure, or an item parented to the figure is selected.

Eg a box parented to a ball parented to the head will take a pose. Even when a box is parented to a ball and a figure is parented to the ball, the box will still accept a pose.

With lights it's even better, a light will take a pose (pz2 or lt2) even if the light is not parented to anything, just so long as there is a figure in the document.

Notes:

If you need to keep files together in one folder, a pp2 (prop) can live in, and work from a character folder, so long as you change its extension to cr2.

When you pose lights via a pz2 you should include an empty figure block in the pz2. I forget why, but I remember that you should.

Using a pz2 on a light instead of a lt2 has the advantage that lights not referenced in the pz2 will not be turned off when the pose is applied, as would happen with a lt2.

If you use a pose on a prop, remember that Poser increments the prop name with a number, a pose for "box_1" will not work on "box_2".

If you add lights to a scene via a cr2 or pp2, the lights and shadow cams should have unique internal names, so as not to conflict with lights that may already exist in the scene.