It took about a week to work out the plugin itself (not counting the Lua design or native coding.) I used NDK to build the .so files, but I wasn't successful integrating that step into Eclipse. I just called ndk-build from the command line. I wrote a batch job to do that, piping the error output to a file, then copy the resulting .so files to my test project in Eclipse, and finally open the error file in notepad.
It took a few days of troubleshooting to build the example plugin from http://appcodingeasy.com/Gideros-Mobile/Creating-plugins-for-Android-in-Gideros. I wasted a lot of time troubleshooting strange errors that turned out to be due to stray non-ASCII white space characters that I picked up by copying the sample code from the web page rather than from the downloaded zip file. Apart from that, that guide is still pretty accurate for the simplest case. That creates a plugin with only static functions, though. If you want your plugin to create a class from which you can create objects in Lua, it gets more complicated.
I tried months ago to build the plugin, but mapping out the functions called in IAB (the in-app billing) plugin was a tremendous headache. It's the kind of code where a function called do_it() calls a method called do_it() which calls a static function in another module called go_do_it() which calls a method called go_do_it() which... Etc. From the Lua call to the native Java function, it's 7 layers of code, and I only half understand why it's done that way. The IAB plugin might not have been the best example to work from, as its complicated further by defining an interface so many different frameworks can drop in to support different app stores. It's clever, but it's not the easiest code to follow. Eventually I had to set the project aside and get back to updates on other apps.
At this point I have very little experience with Android Studio, and I don't know that the impact of switching from Eclipse to that will have on all of this. I appreciate that the goal is to export an APK without needing either Eclipse or AS, though I imagine that will add some steps to the task of creating a new plugin.