After a lot of testing I was able to get hardware accelerated WGL_NV_DX_interop wrking on AMD cards. I have only tested on 7950 and 7990 cards as of this time. Below is the code I wrote for creating, resizing, and rendering a simple test with animating background clear to test the functionality. My access to AMD cards is very limited. Any feedback on devices that work or don't would be appreciated.
One of the key points that I found was making sure the OpenGL render buffer names are created correctly during resizing operations. WPF performs multiple passes at resizing content so this becomes critical.
This uses a share handle on the DX Render Target, this may not be absolutely needed but given you earlier difficulty I implemented it. I have not tested whether or not this code base will work without the use of resource sharing handles. Hopefully it will help you get your product under full hardware acceleration without the blitting operations from OpenGL to DX surfaces. Tried that and its very slow by comparison to DX_interop usage.