The normal setup misses to install the necessary hotplug files to enable users to access the USB mouse devices. I've attached a diff to the kcontrol-3.4.ebuild (v1.4) which enhances the ebuild with the instructions to install the two files for hotplug. Reproducible: Always Steps to Reproduce:
Created attachment 54454 [details, diff] kcontrol-3.4.0.ebuild.diff
Is the purpose of those files to just change permissions on the devices? In that case, it's better to left the task to the system administrator and not force it from KDE. For instance, we cannot say whether the system is using pam_console or not.
See previous comment, reopen if you have more info.