Thanks to XSI Base
member skyPhyr for posting this technique to use HDRI efficiently in XSI.
More information can be found here: HDRI
This leaves us with the question - so what is the truth about XSI and HDRI?
O.K. here goes - this is my interpretation of the situation as best as I've been able to figure it out so far.
1. First of all get your light probe image and make it in to a .map file using hdrtoxsi.exe
2. Then create a sphere in your scene and apply a constant material.
3. Now go in and edit your constant material switching off all toggle switches except colour (i.e. reflection, transparency, incandescense and whatever else is there.
4. Then set the radiance to black (0, 0, 0).
5. Now in the render tree create an image node and use the lightprobe .map file you created. Create a new spherical projection for it and set the mode to implicit (this will just help keep everything hunky dory and smooth, no wavey reflections etc.)
6. now create a scalar share node.
7. now create a pick channel node and link your image node's output to it's input. Open up the channel picker node and choose the red channel.
8. Create a scalar basic math node and connect the pick channel output to input1 of the scalar basic math node. Connect the scalar share node to input2 of the scalar basic math node. Open the scalar basic math node and change the operation to multiply.
9. Repeat steps 7 and 8 for the green and blue channels.
10. Now create an RGBA combine node and link the scalar basic math nodes to their respective channel inputs on the RGBA combine node.
11. Link the output of the RGBA combine node to the colour input of the constant node.
12. Go to the visibility options of your sphere and switch off primary rays (assuming you don't want it visible - if you do don't bother)
13. Delete the default light and create your own for any additional lighting you may want. Perhaps you may only want to add some specular highlights to your scene in which case switch off the diffuse checkbox in your light's properties. If you don't want any additional light then switch off both diffuse and specular - however don't delete the light as it doesn't work without at least one light in the scene. By doesn't work I mean that you will only get reflections of the hdri sphere - you won't get any illumination.
14. Make sure the radiance on all the objects in your scene (i.e. everything except for the HDRI sphere) is set to white (1, 1, 1). Though you can tweak this number if you want to alter the amount of light a particular object receives from the HDRI sphere (note it will receive light from the other objects as well as the HDRI sphere).