*UPDATE*

Since posting this, I've actually begun using the "height blend" node, to perform this function. You could argue my method gives a (tiny)bit more control, but is ultimately inferior in terms of usage and really opens up the door to messy height values. Instead, I suggest taking a look into the "height blend" node as it does pretty much the same thing, but cleaner, simpler, and even outputs a very handy binary mask. I'll leave this here tho for anyone who still wants it. 

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So, here's how I went about stacking the cassettes in my Videocassette Substance.

Here's a graph that explains my process. This is so far the best way I've seen it done.  Hope this helps!

More thoughts:

• Some objects, like my videocassettes needed to be scattered after I had created height, normal, rgb... So, instead of running the above graph per each of those maps (which would be insane) I packed channels as much as possible and was able to perform the above with only two RGBA maps instead of 6, then extracted the results from each after.
• You could run the top layer through a Flood Fill node to create a value/color mask to mask out an object here and there. This would be a great way to keep it from looking too grid-like.
• By allocating more height range towards the upper layers, you can allow for more tilting to create a more uneven surface.
• You're pretty much always going to get a lil bit of clipping depth-wise but that's just the nature of height maps. A way you might get around that is by filling the negative space with something..... maybe some syrup? Whipped cream? Fruit? :P

I used this in my Cracked Concrete material found here.

You may have to DL the image or zoom in super far to read it. Its big. Its also not the lightest method of doing it, but hey, it works. ;)

Let me know if you like this, or if you'd like to know more!

Link in case image doesn't work.

More thoughts:

• You could mask out islands based off their BBox value, and use the empty space for dirt, and mud.
• You could plug in a more primitive Primary crack to influence the sub cracks along it.
• You could use this to create a cool peeling paint material.
• This looks great when displaced across tiles/bricks!
• Use the cracks to drive a tile sampler to place debris, weeds, rocks etc along the cracks' edges.

Have fun!

Creates a stroke from a grayscale shape input.

Here are some functions and results of this tool (any artifacts are from gif compression, not the actual textures):

All of the above were generated through my node and a primitive shape/grunge input.

As you can see, because this instances and offsets, you can crank out a ton of variable patterns, noises, and brushes! :)