Hey- did you ever figure anything out with this?
I've got the same problem. I was having freezing issues with USB Loader GX on some newer games so I updated a couple of the IOSes as you did... now Priiloader won't load, and the Wii won't boot. I was able to get the wii to boot if I put a startup script on the root of the SD card and eject the card before the script can fully run... Then I can do whatever I need/want to do (loads the system menu no problem). Once I'm in the system menu, i can launch the homebrew channel, disc channel, whatever... if the wii tries to reboot at any point (like if try to load priiloader from a forwarder channel or initiate a reboot from the homebrew channel), it just goes to the black screen (or if it tries to reload the system menu; like when you exit a game or the homebrew channel). The only way to get it to come back on is to hard power off, then power back on and eject the sd card after it finds the startup script...
Any and ALL help would be awesome... I didn't make a NAND backup (chew me out for that later), but I know that BootMii is installed as boot2...
Thanks!