Most of the GS rigs I've seen use one-piece alloy wheels.
The wheel I use on my GS rig is 15" x 4", 4 x 4.25 bolt pattern, with 1-3/4" backspace. 60 degree conical seats. 2.9" center hub. Mine's got a negative offset of 19mm but I use one of DMC's adapters. My wheel was supplied by DMC and was one of three styles they offer. But all three are alloy. Mine looks like this--
Recently, a new guy on here posted up pics of his GS rig from Finland and he has a 15" x 6" wheel! I think he sourced it from EZS. I contacted his sidecar builder in Germany and they said they have used 6" wheels on GS rigs many times and have no problem with them even though the centerline is obviously not in line with the centerline of the bike. No unusual wear on the final drive. Here's the only pic I could find--
There's another thread on page 1 where Boxer Metal is whipping up a GS Enduro rig with an EML rear wheel. Pic--
That EML wheel looks like it might be the same as the EZS wheel. Another option is offered by Boxer Metal over on the Vendors page. He'll take your BMW hub and lace it to a 15" car rim. No adapter needed!
So I guess there's more than one way to skin that cat.