Thinking about it, this is a good point. The recommended way would probably to create temporary privilege escalation for the app that needs to access this file, using Pam or something like here:
A user modifiable backlight could be manipulated by malware. If all that is permitted is that a particular app can access that particular file at a certain point in it's code using facilities which have been especially developed to be secure then it becomes much safer.
Extra files can be added to /etc/runit/core-services if you did want to run early, but it would need to be at least 01 because 00-pseudofs.sh mounts sys.