Wow! This is interesting, @cydron, but I’m afraid is too close to what Microsoft did on SkyDrive and fail: the users don’t have a simple way to know if a file is “real” or just a placeholder.
This is the reason I suggested an easier to understand and, thanks to the explanations of the developers, easier to implement. BTSync implemented it well, for what I can see.
May this could be a evolution of the placeholder files? So, we could watch how users react to the feature.