Update on my part of testing:
Yesterday morning i was still playing AC:R while connected to live so it got silently updated. In the afternoon i read the bad news about the new DAE file, when i came home i unplugged the network cable and removed the disc from my console by pressing the eject button while it was still off (powers up the xbox and ejects straight away).
Then i checked xval and it said i wasn't flagged. Then i made a multi backup of my nand with NAND-X.
Just popped in AC:R and got the unsupported disc message and my system got flagged.
I've put the nand backup back that i made yesterday and the flag is gone

.
Now i downloaded the update of xbuc and put it on a testing memorycard.
The game boots fine and nothing is flagged in the last hour after repeatedly powering off, powering on, booting game, removing and reinserting disc etc..
And offcourse i was not connected to live while doing this..
So this may indeed be a temporary OFFLINE solution.
edit reason: mentioned AC:R and AC:B, must be AC:R