Well, I could've saved myself a couple of hours if I'd remembered to do a git pull. This is fixed in master, and the fixing commit was 952ac834 (Rohan Garg, "Simplify bootstrapping of foreign architectures with qemu-debootstrap").
Thanks Rohan! Sorry everyone else for the noise on this report! On the bright side, you can close it as fixed. And I can once again work on a build procedure for live Raspbian images.
