Installer/pkgbuild: Install Newer Files Based on Timestamp?

I have have created an installer that installs an application using pkgbuild and this all works fine.

However, there are some supporting GLSL Shader text files that need to go in a special folder, because we sometimes we update these shaders and users can download a new version and install them manually. Clearly, for a text file, there is no version number like an app or bundle because there is no associated embedded plist.

Is it possible with Apple's pkgbuild tool to install the text files included with the installer only if their timestamp is greater than the ones already on disk?

One workaround would be to install the files from the installer in a temporary location, and then run a bash postinstall script to copy the newer files into the special folder. But I can prepare the installer so that it does this automatically that would be great.

Thanks in advance.