I've got some experience with modeling & stereolithography (design for injection and sand-cast molding of plastic, aluminum and cast iron). The viability of the project would be greatly dependant on the style of the lamp. If it's fairly geometric it should be pretty straightforward. If it's a Louis XIV style with lots of carved sections, that would be very difficult to surface model, so you'd be better off going the low-tech route with wax sculpting & lost wax casting. A 1'x2' Stereolith (or equivalent) is going to be rather expensive (think $800-$1400+ just for the solid lith model - not in the final metal). If you can model the lamp then it can be output to any scale you like, subject to the limitations of your service bureau's equipment.

The two hurdles are: can you model the lamp, and how much will the sterolith/full scale model cost to make. I assume the casting cost would be pretty much the same once you have a model (from an artist or from your stereolith model).

If you can post a photo I can give you a better impression of the feasability/cost/best casting method.

-Zeke