Open the editor, place your player and a MK30 HMG Raised and put gun1 as name (if you don't change any other config an AI unit should be mounted on it). Now click preview, hit esc and put this on the debug console:
player remoteControl gunner gun1;
gunner gun1 switchCamera "GUNNER";
You will have control over the HMG (rather the AI unit mounted on it), in 1st person camera. There's no way to get the fullscreen optics camera from here, numpad 0 or right click do nothing.
Is this a bug or I am doing it wrong?