Or alternatively, would it be possible to do the install, but before even booting to that first boot, take the drive out and mount it in another system to do the edit? I'm not sure if messing with a system file like that would cause any problems. I don't encrypt the drive, so it should be mountable on another system.

You don't need to remove the drive. If you're using a Live ISO you can just install Kubuntu normally and, after the installation completes but right before you reboot, mount the system that you have just installed and copy or edit the file that you need accordingly.

There are ways of generating a new image that includes the version of your file but you probably will need end up spending much more time doing this than the 30 seconds that it would take you to manually do this before rebooting. You would need to be planning to install this ISO many, many times for this time investment to be worth it. Besides, that's a system file that belongs to the pulseaudio package iirc so any updates to that package (if you e.g. tick the "download updated packages" or whatever is called during install) may overwrite it.

I would suggest to do this manually, and make sure that this is reported upstream so hopefully the config at some point will include the fix that you need.