Modify a value inside of a nested dynamic column
After MUCH trial and error... This is the structure required to modify an item nested in a dynamic column.

UPDATE item SET attr=COLUMN_ADD(attr, "level1", COLUMN_ADD(COLUMN_GET(attr, "level1" as blob), "level2", "NEWVALUE")) WHERE id=1;


Which turns;


  { "level1" : { "level2" : "level2_value" } }


INTO


  { "level1" : { "level2" : "NEWVALUE" } }