Just a hint, since you were looking to add i586 - find and sed are your friends. :D
find -name PKGBUILD -exec sed -i '/^arch=/ { /i586/ !{
s/^arch=(/arch=(i586 / } }' {} \;
That will find every PKGBUILD and add i586 to the packages that don't
already have it. Yay!
