As for the point you raise about map synchronisation, an implication of the above would be that mapping SYSTEMMEMORY buffers never blocks, beyond perhaps the draw-time upload.
From what I have seen in real games (e.g. World of Warcraft, Call of Duty Modern Warfare 2) textures are probably more interesting here than buffers. Both games use UpdateTexture with sysmem, D3DUSAGE_DYNAMIC source textures that they later map with DISCARD. When I worked on the command stream I honored that DISCARD flag, but I never wrote tests to show that it is correct to do so.