Copying files from USB during installation using AutoYaST

Hi everyone,
I was wondering if anybody else has encountered this issue and can help me:
I’m using a modified SLES 15 SP1 ISO that contains additional files I want after installation. Getting the files into the ISO is no problem, but copying them to the machine turns out to be a real nightmare.
I finally got it working after a while using the <files> element in the autoYaSt profile. It turns out however, that this only works when using a CD-Rom drive and will not work when installed from a USB drive.
I tried using usb:// and relurl:// which are mentioned in other places in the documentation, but had no luck.
My current solution is to execute a post installation script, (try to) mount all possible drives and look for the file there, but I keep thinking that there has to be a better solution for this.
I am also not yet sure if my proposed solution even works correctly.

Just as a follow-up: looping over everything /dev/sd*, mounting and trying to copy the file from there works.
This still seems hacky to me.