(sorry for bumping oldish thread, but this is the first hit I got when googling)
Had this just happen to me, no achievement at the end of my Insane playthrough. :( But I found a decent work-around, assuming you still have the game running after the credits. There's a "restart mission" option under Load that puts you right before entering the beam. Letting you replay the ending without doing the last mission over.
This probably only works if you don't shut the game off in a fit of rage before. Hopefully it can help someone out there that runs into this as well!