Fortunately, the experiment went very well, and I think kale cubes will be a staple in our house from now on. Here's what I did:
Fill blender with greens, and add just enough water to turn the blade. Blend until it reaches a fairly smooth, liquid consistency (some chunks are fine). I used my Vitamix for this and had to push the greens down with the tamper, especially when I packed in a lot of greens at once.
Pour the kale mixture into ice cube trays, and place in freezer until frozen.
Pop the kale cubes out of the trays, and store in a freezer-safe container.
I found that, in general, one bag of kale (1 lb) filled two ice cube trays when pureed, so each cube really has a lot of kale packed into it. I've been using one cube of kale per serving of smoothie (so four cubes if I'm making smoothies for all four of us).
Any other tips for storing and using greens? I'd love to hear them!