using xinitrc and jwm start files is a good way, but you can use oneshot service too. The place of the created service will depend of the permissions needed for the execution. I will show you the general way to make a oneshot service and how to use it for normal user or root user.
Note : in general way, the name of the service should be created with prefix to avoid conflicts with service installed by pacman. For example, creating a cups service named cups will cause conflicts with the package cups-s6serv. When you create a custom service, create it with a prefix or a no official name like amelie-cups.
Note two : the folder place where create a root service is /etc/s6-serv/available/classic for classic daemon and /etc/s6-serv/available/rc for rc daemon. For user service the folders are /etc/s6-serv/available/user/classic /etc/s6-serv/available/user/rc for classic and rc service respectively.
CPUPOWER : i never used it , but i think this program need root permissions. So, let create a oneshot root permissions service.
create a folder with the service name
# mkdir -p /etc/s6-serv/available/rc/amelie-cpupower
on it create a file up, type with 644 as permissions
# cd /etc/s6-serv/available/rc/amelie-cpupower
# touch up
# touch type
# chmod 644 up
# chmod 644 type
edit the file up and write within the word oneshot. This will declare the service as oneshot service
edit the file up like this
# !/bin/bash
cpupower frequency-set -g powersave
Now you service is ready to use, to enable it for every boot, use s6opts command as root. First, check your current database, If your current database is default, you need to create a new one because linking a new daemon on this database is not alowed.
# s6opts which_db current
this command will return the name of the current database e.g Live
# s6opts update amelie-cpupower
the service should start and declared on the database,so it will be started at every boot.
You can do the same for all your command except that you need to use s6opts with root or user permissions depending of what you need.
This is allow you to apply whatever the command you need on your boot or on your X session when it started because the user supervision tree it's only started when you log on X server.
Remember , we talk about here about oneshot service, not longrun service like daemon.