E.g. have we tested that wglFlush() is sufficient to get an image on screen with a bare GL application?
Yes, it certainly is sufficient with bare GL when blitting / drawing to front buffer, it is just it doesn't happen immediately / before glFlush return. And it seems to me (see the message above) it is quite non trivial at least (even if possible) to replace ddraw blits to frontbuffer with a swap (present).