Re: 0-rank arrays?

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.