I read somewhere, years ago, that if you freeze the beans after soaking, but before cooking, the moisture in them will expand. This has the effect of "pre-softening" them a bit. Another advantage that I see is that you could soak a lot of beans, and freeze them in appropriate sized portions, thus saving the soaking time for each subsequent batch. Just passing this along, though I haven't actually tried it myself; I find my slow cooker does a wonderful job getting them tender, given enough time.