If you want a mechanical movement, something vaguely pseudorandom that uses chaotic behavior similar to the double pendulum idea is the best you are going to do.
Looking at the alethiometer web site, it sounds the hands should go both forward and backwards, not necessarily with each other. One could go forward slow now and back fast later at the same time as another goes back slow now and forward slow later (and forward fast after that). A set of four bar linkages with link lengths affected by cams could accomplish this. There would be a central clock style escapement that would cause things to move and drive a cam and a four bar linkage. The cam would limit, reverse the linkage. The web site shows four hands, stacking up four mechanisms like that is more than.
In the old days, we had these 45 rpm phonograph records with big center holes. If you put one of those on the turntable without the centering device, it played ... interestingly, slow now, fast later. Something similar with the central clock, ticking part combined with a cam that would switch lengths of the linkages would result in apparent random behavior. You get the idea.
Such a thing could be built but you'd need an awesome watchmaker to get all the pieces in something the size of a pocket watch.
Keep in mind Donald Knuth's recommendation that "random [values] should not be generated with a method chosen at random." He was talking about random number generators on a computer but it applies equally well to mechanisms.