I think the concern is that with the induced spin, any loose BP would be forced towards the outside and may not be in line with either the flame from the booster motor or in line with the sustainer motors. The need is to pass flame from the central booster motor to all 5 sustainer motors nearly instantly and in a way that isn't impacted by the induced spin. I can only think of 2 ways that would do it reliably; 1) electronics (absolutely best way) and 2) fast paper fuse.
How would you slice it and install it so that it goes up to all 5 motors in the next stage? It needs to take flame from the booster and pass it into the nozzle of 5 motors in the next stage. I can't think of a good way to do that with a slice of solid BP.
The point was less the shape than the hope that compressed powder from a motor may behave better under rotation than loose powder. I suppose you could use multiple slices side by side, or multiple slivers, or a spider with multiple staged branches.