If anyone is interested I have built a fully working Gradle plugin for Mirah will all of the conventions of the classic Gradle model. This means that source are dropped in to src/main/mirah and test cource into src/test/mirah.
It is on the Gradle plugin portal -
https://plugins.gradle.org/plugin/org.mirah.lang.
The source code is on Github -
https://github.com/ysb33r/gradle-mirah-plugin.
If people who are better at writing Mirah code than me would like to take it for a spin, feedback would be much appreciated.