I replaced fastboot.bin in my local copy of the Select Boot package with eureka's new fastboot.bin. MfgTool was able to start fastboot mode. The linux fastboot tool was able to do "sudo ./fastboot getvar serial" and it showed my kindle serial number.
I was not able (yet) to determine whether it was running fastboot protocol from eureka
's u-boot, or if it loaded fastboot from the mmc (the old way). I suppose that I actually need to sacrifice a K5 (and a K4) by erasing mmc to properly test whether this new u-boot is up to the challenge of recovering a kindle in such a sorry state of affairs.
Also, we need to test other functions in the new fastboot.bin, to make sure that it flashes the correct locations and lengths in the mmc.
EDIT: Even with a few spare K4 and K5 demo kindles, I am (more than) hesitant to erase the mmc in them.