Gosh I finally figured it out. It was just the bsd.rd kernel in the installer that had issues with bwfm0, once I had installed using install77.img and ethernet (although I think you could install offline with the sets on disk and download apple-boot firmware) it worked great. I could bring up bwfm0 in installed system and connect to the internet. I had also tried DFUing to an even older version, 14.5, but it had a similar looking issue IIRC. Maybe this should be documented somewhere?
> To be clear: with the current snapshot, you have everything in place > under /etc/firmware, just like the installer put it there, right? > In particular, you have the /etc/firmware/brcm files, right? /etc/firmware/apple-bwfm has the bwfm0 files for me. > Apparently, that's where they are supposed to be found. I guess it differs from release, or they changed it to apple-bwfm so it's less confusing and doesn't conflict with the USB broadcom WiFi stuff.