In order to display the diff of the build scripts, pacaur could perhaps check the last installed pkgver (if any), and go back in the git history of the AUR repo until it finds the commit corresponding to that pkgver (assuming there's only one such commit, which "should" be the case for "well-behaved" repos), and use that as base for the displayed diff.
Version
pacaur -vDescription
In order to display the diff of the build scripts, pacaur could perhaps check the last installed pkgver (if any), and go back in the git history of the AUR repo until it finds the commit corresponding to that pkgver (assuming there's only one such commit, which "should" be the case for "well-behaved" repos), and use that as base for the displayed diff.