I dont think that would be possible - as i said, i want a 28-25-23 ...13 (or even a 28-26-24...14) and shimano dont make that, as you said, the sprockets are riveted to a carrier as a 28-24-21 or 27-24-21.
I particularly want narrower gaps from 28 down, the 13 is to make sure there arnt too big a gaps further down the cassette.
Now I'm not familair with modern Shimano cassettes, but IF the smallest sprocket has the same fittings as larger sprockets on the cassette hub then you can make one up:
No you can't all Shimano cassettes have a number of the larger sprockets - at least 3 but up to 7 - that are bonded together so as to provide the extra stiffness required on such large diamaters without a high weight. The options are Miche where they're all seperate or suck it up...