I can't say much about the details of 0-length arrays. I suspect that the 'real implementation' of HDF5 dimension scales will not need a user visible attribute. It will probably have an API for retrieving the information, along the lines of how the dataspace API works. Since the new design hasn't been put forward, it's difficult to speculate. The main point is that I wouldn't expend a lot of effort on the details of supporting dimensions at this time.