Is there any available software which can be used to set the
/proc/<pid>/oom_score_adj of processes after they're created, based upon the executable name and the user/group?
I have created an utility that does it solely based on the executable name, in addition to setting the CPU and I/O priority: https://git.janouch.name/p/desktop-tools/src/branch/master/priod.c
It can be installed with the parent package as per its README. Just copy the example configuration to /etc/priod.conf, change it according to your needs and enable the systemd unit, and it should work. Note that it goes through already existing processes on startup, too.
I hope it will help someone, at least as an example.