Summary and Info:
The archive format and the used libraries (libarchive) already support mapping of usernames to uids.
If the user is created as part of pre_install, it can be used as owner for files in the package.
This is also true for groups.
Only makepkg is missing a way to add those users temporarily during package creation so that you can chown the files and bsdtar can pick them up.
The following project allows to create arbitrary users and groups which can be used inside a fakeroot environment.
https://github.com/progandy/fakeuser
It is a library for LD_PRELOAD and a small c program to write entries to a custom group or passwd file. The directory for the custom files is specified using an environment variable set by the libarary on first load for all children.

Maybe also reserve 5 user and 5 group ids to use for this purpose in order to avoid duplicate ids in some rare corenercases.