Some thoughts on this:

== PKGBUILD

ghosts=(var/cache/foo.db
var/cache/bar.db)

== SRCINFO/PKGINFO

ghosts = var/cache/foo.db
ghosts = var/cache/bar.db

== /var/lib/pacman/local/foo/files

%GHOSTS%
var/cache/foo.db
var/cache/bar.db

== Some questions about this:

1) Is pacman allowed to remove these files or only when forced to?

[Typically generated files can be important so a new flag is probably the
wisest choice here.]

2) Could this be integrated with pacman -F to directly list ghost files?

== My opinion:

It seems like this feature would be mostly only used for pacman -Sii, -Qii and
-F with pacman itself not being involved in their management, only tracking
their existence.