The hotfix has fixed the exclusive access problem -- that is, I can burn DVDs with ImgBurn with the exclusive access option checked and it works fine now, every time.
However, I am worried the hotfix may have introduced other problems. I leave the PC running 24x7 and for 2 days straight now since I installed the hotfix, I have sat down to the PC to find it locked up -- screen is asleep (does not wake up), no mouse/kb response, nothing -- I can ping it, but cannot map drives from remote PCs. All I can do is reboot. It is possible it may not be the hotfix, but this behavior is new and I've made no other changes. I am giving it a few more days to see if it persists. If so, I will restore my system backup (without the hotfix) and see if the problem goes away.