VNC Meta key not working with realVNC server+viewer android on Mac OS X

I'm using realVNC server on Mac OS X (VNC® Server 5.3.1 (r17370) x64) and VNC viewer on Android from realVNC. I use emacs in Terminal and iTerm2, where I normally set option to Meta, but this does not work with the soft keyboard 'option' keys in the android app. Nor does it work with the Alt key or any key on the Hackers Keyboard app in Android.

Now obviously I can just ssh to use any console application, but I would like realVNC to work properly. I've read that somewhere there are keyboard parameters, and that maybe the option key is not sending OPT_L, but rather some other code, so it's not recognised by the terminal emulators... perhaps that's that's the problem?

How do I send the Meta key?