Well as long as it ships with an installer, the above method still works - just omit the build step altogether (do a chroot if the installer doesn't offer a choice of where to install). If it doesn't have an installer, but comes self-contained, then just `cp $srcdir $pkgdir/opt` & call that good.

And if you're dealing with a non-source package that doesn't have an installer and isn't self-contained, then installing/packaging it is difficult on any OS - not just Arch!

For reference, here's a PKGBUILD for a non-source, proprietary application. It's pretty much as above, but includes some fancy .desktop files & such, too. https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=disco...