If you get to a modded recovery image, you can run adb
1. Run "adb shell".
2. Once you're connected, remount the microsd card (it the os is broken it doesn't mount it automatically, at least on my phone) by running "mount /dev/block/mmcblk0p1 /sdcard"
3. Run "exit" on the terminal, so you can go back to your desktop computer.
4. Copy an update file to the sd card using adb, for example: "adb push [name_of_the_file] /sdcard/update.zip
5. Run the update from the recovery menu.
If that doesn't work, and if you did a nandroid backup, you still can update using fastboot by using the image files from the sd card (wich, by the way, you can copy from the phone back to the pc by using the command "adb pull [remote_file_name] [local_file_name]