I LOVE iwagumi style. I thought Amano came up with the style though? You could do that with a NPT, you would have to have layers of tiles under the substrate to build up height and different levels, you can't have the substrate to deep. Also because you use a small shallow rooted ground cover the nutrients need to be close to the surface (in my tank with glosso I mix sand and Amazonia together.) To take up the nutrients you will need floaters like frogbit, this is what Diana used to take up the waste in her tansk with only ground covers. Doing a DSM will be easiest I would think since it gives teh glosso a chance to get established before it is flooded.
Glosso is probably the best choice, HC doesn't grow all that well without CO2 while glosso will still go crazy.