If NEM would use POS, the smallest possible importance would be 1 / maximum number of harvesters. Because you have ~9lbn coins and you need 10k coins to start harvesting, this means that the maximum number of harvesters that can exist is 0.9mln.
The smallest importance is 1/0.9mln which is approximately ~ 1.11 * 10^{-6}.
But I am not sure how much does this change with all this NCDaware score.
I don’t think it wouldbe that in POS either. You don’t need to be harvesting to have “harvesting power” i.e. importance in nem. So it’s more like 1 distributed among all none 0 importance accounts.
The onnly difference is how that is distributed. In POS it is entirely based on balance, in POI there are other factors as you correctly point out.
I can’t tell you what the number actually is though simply because I don’t know what the smallest “unit of importance” is.
Rereading my original post, I see that my explanation about the number of harvesters is pretty confusing. I tried to edit it to make it more clear.