After evaluating all of the options, including playing around with some samples of a variety of aluminum alloys, I decided to go with an aluminum desktop as originally planned. I'm pretty optimistic that we'll love it. The runner-up (or Plan B, if it comes to it) was an $80 brushed aluminum laminate countertop from IKEA.
I ended up buying a 6-foot sheet of 3003 aluminum
, 0.063" thick, from Airparts, Inc. in Kansas City. I've cut it to size and earlier tonight bonded it to a 3/4" sheet of Baltic Birch plywood (13-ply) with contact cement. Tomorrow I plan to glue a 5"-wide second layer perimeter to the bottom of the desktop to increase the total thickness to 1.5" and 25 plys.
Here are some photos. The desktop measures 59"W x 27"D.