Thinking of doing this x11 specific, I think that maybe we don't even need a special boolean flag if your MR !2217 gets in? Once we have a separate tracking of internal pixel format in WGL_WINE_pixel_format_passthrough we could just always fail setting internal pixel format once external is set. The extension itself will still work as if an app wants to use GL together with d3d it will be able to set GL pixel format (which will result in wined3d creating a backup context).
I think that'd work, yeah.