Good news. I don’t know if it’s attributable to (my) user error or recent update to macOS 10.13.5, but my Oppo HA-2 and bcm2835 ALSA IEC958/HDMI devices are working well. Thanks!
The ALSA device isn’t providing “UltraHQ” quality over HDMI as DietPi implies, but that’s probably to be expected given its Linux kernel limitations. I think the so-called “UltraHQ” refers only to reimplementation of the original onboard 3.5" analog jack PWM-based 11-bit audio @48kHz as 7-bit 2nd-order Sigma-Delta modulated at 781.25kHz, as described here:
https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=136445&hilit=audio+quality
Appreciation to @john and @support. Roon is working great for me!