Can I veneer over metal?
Yes you can, and the most important thing to remember, as with all veneering projects, is to get a good bond between the substrate and the veneer.
How do I prepare the metal substrate for the veneer?
Metal is not a porous substance, so glue will have a hard time adhering to the metal. Totally sanding (or sandblasting) the metal surface with a high grit sandpaper to remove any existing finish is necessary. It also creates a roughness that will provide a better surface for the glue to bind to.
What glue do I use to glue the wood to the metal?
Epoxy or polyurethane glue are two possible glues that you can use to veneer over metal. If you choose to use an epoxy adhesive, make sure you use one of high quality (not a five minute curing time for example).
Any other concerns about veneering over metal?
Depending on your veneer choice, epoxy may bleed through the veneer. A way to avoid this is to glue a veneer backer on to the metal substrate using epoxy. Basically this is a low cost veneer that you will cover up with the good piece of veneer you intend to use. Then after you have applied it to the metal surface, you can then use regular wood glue to glue your final veneer over the backer. Just note that this will provide an extra thickness as you will have two layers of veneer instead of just one.