if i remember correctly, the option of downloading the update and choosing to install/flash it later exists.
should leave an update.zip on you sdcard or in you file system somewhere. now if you could take that
zip and do some root voodoo to it then flash it that might work right? you are rooted so you have a custom
recovery so you don't have to be concerned about signatures because you can su in terminal and dd your
image appropriately or in some instances use root browser to replace files manually.......and then if you post that
tasty update for consumption......also the should be an ota block in you partition scheme,maybe that's useful