Switched to APKs from Alpine edge for the binary seed and added a SHA512SUM of the distfiles which are now fetched. Alpine edge is of course not a stable thing but at least it now means it's not just built for my machines and probably should work for other architectures as well.
Also even for me it's a great change because straight up unpacking the APKs with tar(1) is a pretty nice time saver.
Here's the repository URL again now that other people should be able to play with it: https://hacktivis.me/git/bootstrap-initrd/