I've been using and tracking Stefan Dossinger's work re: DDraw over WineD3D and decided try and find out if IWineD3DSurface was far enough along to be able to be used in place of IWineX11Surface for 2d applications (Hopefully to eliminate depth conversion issues).
Short answer, no.
Long answer, Screen displays blank, trace log shows that glReadPixels at line 615 in wined3d\surface.c (LockRect) is failing with error GL_INVALID_OPERATION. Further investigation reveals that this is because glFormat is GL_COLOR_INDEX, yet the display is an RGB visual (presumably because an indexed colour visual was not available, and this buffer is connected directly to the screen).
I figured I'd look to the people already familiar with the WineD3D\DDraw codebase before I tried to figure out my own solution.