Lessons learned so far from the experiment:
1) a 2-inch square of cloth in the middle (as required by the design) does not protrude through the blade edges (at least with the sort of wedge design they were using). I was nervous about this; now I'm not.
2) car-cleaning sponge is probably too soft. It does however collapse nicely. Its also not bad to cut, but the sander will probably shred it. So I'll need to look for another tip material.
3) Getting a point on a wedge is a bitch. A long, leaf-shaped spearhead would be easier to cut, fit nicely within the minimum dimensions, and probably work quite well.
4) From poking myself with it, it doesn't feel too bad. But it will probably feel quite different on the end of a 2m pole, with all that extra mass behind it.
5) Most important $2 shop blades are cheap shit, and nowhere near as sharp as the more expensive ones you get from Mitre 10. The sander may be able to clean it up, but I'm thinking it may be worth paying four times as much to get a decent smooth edge.