Is parameter overrides supported in ROS2 launch?

edit

As a workaround, I periodically update parameters like this, but I know it's not a good way...

sample.launch.xml

<launch>
    <include file="my_launch.launch.xml" />
    <include file="parameter_overrides.launch.py" />
</launch>

parameter_overrides.launch.py

from launch import LaunchDescription
from launch.actions import ExecuteProcess
def override(node, param, value):
    return ExecuteProcess(
        cmd=['watch', '-n10', 'ros2', 'param', 'set', node, param, value],
        output='screen',
    )
def generate_launch_description():
    return LaunchDescription([
        override('node', 'param', 'true'),
    ])

I'm not sure I 100% understand your question, and this may depend on your version of ROS2, but the following may be helpful:

In ROS2, parameters can be set in yaml files and can be scoped to specific nodes (e.g. from https://roboticsbackend.com/ros2-yaml-params/)

/node_1:
  ros__parameters:
    some_text: "abc"
/node_2:
  ros__parameters:
    int_number: 27
    float_param: 45.2
/node_3:
  ros__parameters:
    int_number: 45

To me, this feels like what you are looking for... You can also set generic parameters (that would apply to all nodes) by using ** instead of the node name, e.g.:

/**
  ros__parameters:
    int_number: 5

You can also set parameters directly from the commandline (see here) using ros2 param set <node name> <parameter name> <value>