Two stage collapse using rigid body physics? I have long wanted to make a two-part destruction using RBP, namely, an object is struck and breaks into two pieces which fall, hit the ground, and then each break into multiple smaller objects.  

My solution works rather well, though it can be tedious.  There may be a better way which may well be the Bullet Constraints Tools but I have had little success in managing the details.  A short test movie can be found here. Does anyone know a better way?
Unfortunately blender doesn't really have much support for this kind of thing at the moment (the rigid body simulator itself was just added fairly recently). Ideally the fracturing would be done real-time, based on impact forces etc.

However, it's possible to fake it with constraints, as you mentioned.
I like use this addon for setting up lots of breakable constraints between the shards.

There are a couple ways you can go about this.

One way is to fracture object object into lots and lots of little shards, and constrain everything together with breakable constraints (using the addon).
Video of result, and blendfile.



Another way is to fracture the object normally, then manually cell-fracture the bigger shards and constrain them together with the addon.
Video of result, and blendfile