This feature isn't built into Pkgbuild, but after Googling around I solved it with the help of Greg Neagle's blog post:
http://managingosx.wordpress.com/2012/07/05/stupid-tricks-with-pkgbuild/
1. Create a file in the directory of your Makefile named PackageInfo containing the following:
<?xml version="1.0" encoding="utf-8" standalone="no"?>
<pkg-info postinstall-action="restart"/>
2. In your luggage Makefile, the first three lines should be:
USE_PKGBUILD=1
include /usr/local/share/luggage/luggage.make
PB_EXTRA_ARGS+= --info "./PackageInfo"
Another way to do it would be to change your package so that it uses PackageMaker, and override a setting PM_RESTART=None. But I couldn't find documentation on the opposite argument to "None"...
Cheers, Graham