Any tips on how to run script way early in boot on linux?

Problem is that i need to push text string into driver_override on pci bus, before driver has been loaded.

I did solve this using systemd earlier, but after a update driver loads before systemd even starts so no luck there.

Also driver doesn't support unbinding.

Any ideas ?

You can add an init=</full/path/to/script> kernel parameter to your kernel cmdline to run your script instead of systemd's init. Then launch systemd's init from your script.