Ok.
Ran the talker app directly (assuming this comes from https://github.com/ros2/tutorials/blob/master/rclcpp_tutorials/src/topics/talker.cpp], and can see the Hello World output.
On another terminal, run ros2param set talker/foo 1, I get the response Failed to set parameter.
This is running beta1 with FastRTPS as the only rmw, on a Windows machine.