Developing a custom staging rule is possible, provided you have a valid Nexus Professional license.
First follow the steps for Accessing Sonatype Nexus Private Development Repository.
Then, check out the branch of the nexus example plugins project that matches your version of Nexus.
Finally, build the resulting project using "mvn clean install". When it is complete you will find the staging rule bundle (a zip file) in the nexus-example-plugins/pro-plugins/stagingrules/target directory. Unzip this into sonatype-work/nexus/plugin-repository, and restart the server. The new custom staging rule will be available.