Well, it's been a while since I did anything with a "halo effect", but I can tell you how I did it in poser the last time I did it.  I used a simple box primitive and placed it immediately behind the character in the scene.  Then I did a render and went into photoshop with that render, filling in with black everything that wassn't my box primitive (fairly easy to do, the primitive was white so I just selected by color to get the primitive selected, then selected inverse to get everything other than the primitive selected, and filled with black.

Then I switched to a grey color (started with about 80% black to white ratio) and painted sort of a halo around the black edges of my render.  It doesn't have to be very accurate or even all that pretty, I did it with a fairly soft brush and just went around the outside edges of the black portion where my character would be once the scene was rendered.  Once your finished with your halo select all the white that is left over and change it to black as well, so the only thing you have in your image file that isn't black is just the halo you painted.

Save this file as a jpg and go back to poser, load up your scene again.  Select your box primitive and go into the material settings, load up your jpg file in the material editor and connect it to the transparency portion of the primitive, then set the transparency strength to 1.  

This will cause the box to become more or less invisible, except for the halo you painted, which will allow light to still pass through it from behind but it will change the color of the light based on the specular color you've chosen in the material settings for your box primitive.  You can add whatever specular color you wish for your "lighting" to be, I believe at the time I used sort of a gold color - it was to place a "halo of light" around a space vessel that was crossing in front of a sunrise on a planet as I recall.

If you need to make the halo less subtle and more obvious, open your transparency jpg and increase the brightness settings and contrast settings, then resave.  You want your background for the transparency to remain black so only the halo will be visible in your render, the rest of the box primitive will stay invisible as long as the rest of your transparency jpg is black.  Conversely if you need the halo to be more subtle adjust your image so that it is darker, the darker grey your halo painting is the more light it allows to pass through and as such the less noticable it becomes.

You can also get some really spectacular effects for your halo by playing with the reflective and refractive properties it it possesses in the material room. 

While using the transparency on a primitive like this might seem a bit more time consuming than just playing with some lights, it was the only way I could find to get what I felt was a sufficently decent halo image myself using poser.  

Hope that helped,
Stepdad