When portage extracts precompiled binary tbz2 archives, files will get owners according to uid, which might be different that on the build machine. IMO user names should be used as a better/safer solution.
Checking the code I found the following observations. The creation of the binary packages is done in ebuild.sh in the dyn_package function. The archive is created with 'tar cvf' which should by default preserve user names and not user uids. The extraction takes place in portage.py in the pkgmerge function. The archive is extracted with 'tar xpf' which also should preserve the user names. According to the tar man page numeric uids are used when the --numeric-owner option is provided. Maybe the -p option silently includes it. Someone any comments?
Pretty sure you're right, Geert. Looks like a "worksforme"