After loading text, data and syms, the screen switches modes and becomes unreadable. After booting completes you can make out a blue background and whitish stripe across the middle of the screen which would be the first menu.
By booting into OS X I determined that the iMac has a Radeon GPU.
I was able to get around the problem as follows:
1. After booting to the first menu, type Ctrl+Alt+F4 to get to a shell. The screen is still garbled here, so this alone does not help.
2. Blindly type "kldload radeonkms". The screen then resets to a readable state.
I have not tried 11.2 on this machine, but would be willing to if it might help isolate the issue.
Must use the same trick when booting from installed system.
To fix add the following to /etc/rc.conf:
11.2-RELEASE exhibits the same issue.