This is a specialty model of a luggage rack with framing arranged to carry a spare tire. These have been appearing on eBay a couple times per year. For as long as the ad lasts (ending Jan 1, 2007) you can see it here:

http://cgi.ebay.com/eBayISAPI.dll?ViewItem&item=180067484446

There is a shop in England that may make these up for you on special order with any variation you might like. Polished stainless steel may be less expensive than chrome. Check here:
Sports Car Metalworks, Duttons Farm, Bangors Rd South, Iver, Buckinghamshire, SL0 0AA UK - Ph: 01753 654144   Various MGA and T-type panels and luggage racks.