Is it possible to extract juice from these vegetables using a blender? How much would it take to produce 100mL of juice?
Baby bok choy:
Since they're mostly water, wouldn't pureeing them in your blender essentially give you a pitcher full of thick, green liquid? If you didn't want the vegetable matter and fiber, straining that through cheesecloth or coffee filters would give you something of juice consistency.
How many would make 100 ml would vary depending on their individual size and water content if you're straining out the vegetable matter and fiber. If you add a few at a time until you're at the right liquid level you'll eventually be able to eyeball the right amount or at least within range based on how the bunches look when you buy or go to use them.
A auger juicer would do a better job, but a short stint in the blender will do
100 ml of water is 100 gm (isn't the metric system cool!). So expect to get around that
I would imagine the bok choy would be disgusting and slimy, but the cabbage should be great!