Okay, as promised. I bought a new SLX cassette for this butchering, but there's no reason that you can't use your worn out MS cassette (they all appear to use the same spider attachment, though I have not verified this).
Step 1: Drill out the rivets on the two smallest sprockets (they're already mostly center punched) using a drill press. You can do the others if you want to make your life easier later.
Step 2: Punch out the rivets with a 1/8 or 3mm punch. The rivets are quite tight regardless of whether or not you remove the head all the way.
Step 3: Cut off the arms of the spider in a convenient place. I chose the second hole, and I may still remove them all the way down to the first one.
Step 4: File/cut excess, and tap the holes (use an M4x0.75).
NB: you can through drill and use an M4 nut on the other side, but the clearance is tight.
Step 5: Acquire a coaster brake sprocket of your choosing. I had a 20t Alfine sprocket with integrated chainguard, which got this whole thing rolling around in my head. I noticed that the holes for the plastic guard seemed about the same spacing, and reasoned that rather than changing their tooling, they would re-use what they could in manufacture. As you can see, I was 100% accurate in my size assessment, even if the reasoning is not.
Step 6: Using a file/sander/stone/milling head, enlarge the center hole of the sprocket to fit over the raised cone on the modified spider. If using a different sprocket than the above, fix it with a clamp/spray adhesive/whatever, and center punch it through the rear holes of the spider. Drill/enlarge the holes to 4mm.
Step 7: Mount to spider with hardware. I used road brake retaining screws--please note that there is not a preponderance of material that has threads on the spider. I mocked this up as is, but will likely go back to install the screws using red loctite. I'm still personally mulling over putting nuts on the back.
Things I didn't immediately think of at the outset:
-Freehub spacers to take up room are not really readily available yet--regular HG spacers fit (the 'splines' on Shimano HG spacers have the same ID as the outside diameter of the Microspline freehub), BUT
-The lockring is made to clamp down on something SMALLER than the diameter of the splines on the freehub. You must either use the top two sprockets, or custom tooled hardware will need to be made.
At any rate, this is currently a proof-of-concept. I need to get checked out by the doctor before trying this out, as my back hasn't recovered from 1.5 weeks ago pulling it while riding.
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