...Conversely, i suppose the charged molecules might be able to migrate through the porous paper, and reduce the potential difference....
....If the ferrocerium rod is kept well away from other objects (except for its red lead to the meter), and the black lead is simply connected to ground (i.e. without the carbon electrode) is there still a potential dif Usually if there is no 'free passage' between working electrode and counter electrode there is no voltage.
Usually if there is no 'free passage' between working electrode and counter electrode there is no voltage. I use nylon fly-screen about 0.5MM thick or a couple of pieces of paper between electrodes and see a voltage. If there is direct contact it is a short circuit effect - voltage drops to zero but then recovers often over a moment or two. With this particular material recovery from a brief contact is almost instantaneous, and remains so even after a dozen or more trials.
Ferrocerium is an outlier compared with the behaviour of other LEC materials in some way I don't understand- but it has shown me in the past that it is a very good cold fusion fuel with a considerable appetite for hydrogen.
As to your other question about earthing, I have not tried it with ferrocerium, but have with other materials. Normally you need a counter-electrode (which can be connected to earth - done that)- but this is such an oddity that maybe it will show the 'sound of one hand clapping.