I hacked something together last Friday which does what you want: Declare ArchUnit tests in one place and allow you to run them somewhere else: https://github.com/gradle/gradle/commit/0789fa1bfe513d2536b5aaf3cca736f3c20c94f7
The basic idea is to get the classes variant of the project which declares the tests and add them to the arch unit test task in the current project. Note that Gradle needs the classes, not the JAR to actually run the test.
Also note that the changes are mostly a spike and still need some more love.
