multithreaded rendering on android breaks nativetexture update in plugin

For my ImagePicker for Android plugin I started working on implementing native texture updating. It works perfectly now and is a lot faster then saving / loading through temporary files. However in multi-threaded rendering, the GLES20 code still works (or returns 0 errors after each call) but the texture never gets updated.

This is of course due to the rendering thread being in an entirely unknown state. However there seems to be no way of triggering a plugin function to happen at the right time in the rendering thread (except for using update and running in single threaded rendering of course).

Implementing void UnityRenderEvent (int eventID) into a JAR plugin does not seem to be a viable option (no documentation available).

As the eventID is shared amongst all such plugins and no sense can be made from the documentation. It seems to also require a C++ plugin, as no other examples are given.

Does anyone know of a way to solve this issue? Ideally there would be a special coroutine, function or thread we could just implement or inject to. Or even flag the texture to lock then wait for the proper gl state.

GLES20 compatible solutions only please, as I have currently no GLES 3.0 android devices available. So using Sync or Fence classes and functions is not an option.

I have tried using commands like glFinish, glFlush. before, after and both around the GL code block. With no effect to either result or errors.

Related forum thread: http://forum.unity3d.com/threads/gl-issuepluginevent-in-android-java-plugin.277066/