You can create a recursive custom block to create a JSON list.
(You can drag and drop this image into the editor to import the block; if it doesn't work, click on the image to blow it up and try dragging that)
Once Snap! 10 comes out, you can program the default [JSON] OF [] block to do this.

Supports 1D, 2D, 3D, etc. lists, text, booleans, and numbers, just like the [JSON] OF [] block.
Also escapes any character that will unintentionally alter the JSON.
Essentially the same thing without the formatting.