Well I got it working.
The problem turned out to be that the "Apple CD/DVD Driver" extension was disabled. Enable it -> problem goes away. Disable it -> problem returns.
I discovered this by digging up DVD Player 2.0, which installs without complaint, but upon execution complained about that extension being missing. Once I enabled it, DVD Player 2.7 installed without problem.
The problem turned out to be that the "Apple CD/DVD Driver" extension was disabled. Enable it -> problem goes away. Disable it -> problem returns.
I discovered this by digging up DVD Player 2.0, which installs without complaint, but upon execution complained about that extension being missing. Once I enabled it, DVD Player 2.7 installed without problem.