Summary and Info:

AFAICT neither makepkg(1) nor PKGBUILD(5) says that makepkg *MUST* be run in the directory where PKGBUILD (and changelog, install) is. in fact, PKGBUILD(5) includes

install
Specifies a special install script that is to be included in
the package. This file should reside in the same directory as
the PKGBUILD [...].

changelog
Specifies a changelog file that is to be included in the
package. This file should reside in the same directory as the
PKGBUILD [...].

Steps to Reproduce:

roman@wrench ~/tmp 0 1090 0 > tail fubar/*(.)
==> fubar/PKGBUILD <==
pkgname=fubar
pkgver=42.69
pkgrel=1
pkgdesc="makepkg -p canary"
arch=('any')
url="http://example.org/"
license=('BSD')
changelog=$pkgname.changelog
install=$pkgname.install


==> fubar/fubar.changelog <==
omg

==> fubar/fubar.install <==
post_install() {
echo hello world
}
roman@wrench ~/tmp 0 1091 0 > makepkg -p fubar/PKGBUILD
==> ERROR: changelog file (fubar.changelog) does not exist.
==> ERROR: install file (fubar.install) does not exist.