You'll recall that back in the day AMP was a purveyor of crate rear ends. I have a 1993 Rocky Mountain "The Edge" (nod to U2?) in my bike cave that needs assembly. The rear end is of course aluminum Amp stays and shock. The front triangle was welded by Mr. Dekerf AFAIK. Yes it is the steel one, the '94 went to aluminum front triangles.
With the integration of todays bike I just don't see crate rear ends being a think and I don't see big manufacturers wanting anyone else to use their designs. Too many liability issues and so on these days plus they want people to buy their whole bike, not just a rear triangle.