Is it possible to invoke zoom focus assist when using a non Panasonic auto lens on a GH3? This function is very useful for precise focus adjustment. I would like to be able to use it with any kens.

Two ways:

If you have touchscreen enabled simply touch the screen, that should "invoke" it. Works on mine with a Nikkor AI-s lens mounted.

Or...

Press left arrow or right arrow on the back dial, an indicator showing the focusing area will appear. Position it with the arrow keys then press "Menu/Set" button, zoom focus assist will "kick in",

With both methods, a light press on the shutter button returns to "normal" view.