The only thing I can imagine would be relevant is that I'm using pfn's android-sdk-plugin.[1] I also have a sort of generic build.scala file I tend to reuse for Android projects (it's based partly on Ascension's, but has been tweaked a bit), which may also be useful.[2] I leave most of the proguard configuration up to pfn's sbt plugin since it does an acceptable job, but your mileage may vary.
[1]: https://github.com/pfn/android-sdk-plugin/
[2]: https://gist.github.com/nilium/db5bfac2174370ebdd97