This works:
]$ ros2 run demo_nodes_cpp talker talker:__node:=new_node_name
[INFO] [new_node_name]: Publishing: 'Hello World: 1'
^Csignal_handler(2)
As does this:
$ ros2 run demo_nodes_cpp talker __ns:=/new_ns
[INFO] [new_ns.talker]: Publishing: 'Hello World: 1'
[INFO] [new_ns.talker]: Publishing: 'Hello World: 2'
But this doesn't seem to:
$ ros2 run demo_nodes_cpp talker talker:__ns:=/new_ns
[INFO] [talker]: Publishing: 'Hello World: 1'
[INFO] [talker]: Publishing: 'Hello World: 2'
^Csignal_handler(2)
There's a chance this combination isn't expected to work, but I think it might be.