The rc.local idea did not work as mythbackend is setting the high volume when it starts recording with that device, regardless of my volume setting for playback in frontend using the mixer.
I found a solution though. I do it in the change channel external script that the backend calls when changing channels or starting a recording on that tuner. But because there is a delay when it sets the volume, I had to build in a delay by using 2 scripts with one returning immediately so the frontend response is not slowed down. Then after the timeout of 5 seconds (which seems to cover most cases) the 2nd script lowers the recording volume.
Here they are, starting with the master script that is placed in the change channel external command field in the card setup for mythbackend:
Hopefully this idea will help out others.
v4l2-ctl -c volume=57000 -d /dev/video2