So, this is more involved than you probably want, but an early (really, don’t use it quite yet :)) version of our Google Play services integration can be found at:
https://github.com/EpicGames/UnrealEngine/commit/122667ec999c55b8369c42e25bc4b473cfa5239f
Some of the key changes related to adding a .jar are in UEDeployAndroid.cs, where we copy the Google Play services library to our Intermediates, and then run ‘android.bat update’ on it.
Other important files are AndroidJNI.h/.cpp for calls across the C++/Java boundary, and AndroidManifest.xml, where we let the Android SDK build system know that we’re integrating the Google Play services library.
There may be a simpler approach for what you’re looking to do, and if you find something that works better, please let us know.