If you can put a piece on either side of the block and equalize them, then weight on the anchor should result in balanced forces and no movement of the block. I have actually done this, though the block was never weighted, so I don't know if things would have held as I expected, but it makes sense to me.
Cams, because they have an expansion range (duh), would be my first choice for an expanding crack. Passive pieces have a very small expansion range (double duh), so if the crack enlarges even slightly, they will fall out.
I think a good mathmagician (I'm not one) could calculate the outward forces generated by a stopper with a given taper angle versus a cam with a given cam angle. I know that metolius cams generate more outward force than BD, or any other cam. This gives them better holding power, but smaller range.