I've seen a few, mostly on Toyotas and F*rds.
If I'm thinking correctly you want the rear hoops to mount your shocks on. A true bed cage would be sorta useless without a cab cab to tie it and the frame into. Here's a photo I found on a Toyota BBS user's web site:
The link to the web page is http://www.home.earthlink.net/~orvacian
Very simply, you just make a bar that goes across the bed, and two "hoops" or bent tubes that go through the bed and tie into the frame through either welding, bolts, or a combination of both.
Then you get some pre-made tabs from places like Fabtech and weld them on for the mounts for your shock eyes, usually heim joints for the long reservoir or "pre-runner" shocks. You can lay the reservoirs on the crossbar and use those metal hose clamps to tie them down.
Cut holes in your bed to get the shock through and allow enough room for forward and backward travel. Pay close attention to how you mount the shocks on the bottom, so that you don't end up getting them too low on the axle to cause some wrap problems and possible heavy rock damage!
I think that's about it? I don't have a photo right off hand that I could show you; I'm doing this all from memory. My buddy with the Toyota loves his set up though. I'm not sure I could cut through my bed that easily!
Along the same lines, I saw a guy with a full size Chevy that had the full suspension set up done through Fabtech and also a 3" body lift. The body lift gave him enough room to create a smaller bar and hoop setup to allow his shocks to stay under the bed (so he could carry his sand toys). To tell you the truth it made much more sense, as the rear springs only got up to about 10" of travel (with limiter straps) and they were damn good springs (Deavers). He had the lay the shocks down a little bit but that didn't affect anything much.
Hope that helps.