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I ended up using a custom Substitution. Not sure if that's the cleanest solution, but it worked for me.

class ConditionalText(Substitution):
    def __init__(self, text, condition):
        self.text = text
        self.condition = condition

    def perform(self, context: 'LaunchContext') -> Text:
        if self.condition:
            return self.text
        else:
            return ''

Putting everything together:

def generate_launch_description():
    use_prefix = (EnvironmentVariable('SOME_VAR', default_value='0').perform(LaunchContext()) == '1')
    n = launch_ros.actions.Node(
        package='my_pkg',
        executable='my_node',
        prefix=ConditionalText('some_prefix', use_prefix),
    )

return launch.LaunchDescription([n])

Edit: Just stumbled across a simpler solution in Gazebo's launch files: https://github.com/shiveshkhaitan/gazebo_ros_pkgs/blob/a6e395195bc878a76200fb948cc8cfce414b98fe/gazebo_ros/launch/gzclient.launch.py#L82

def generate_launch_description():
    use_prefix = (EnvironmentVariable('SOME_VAR', default_value='0').perform(LaunchContext()) == '1')
    n = launch_ros.actions.Node(
        package='my_pkg',
        executable='my_node',
        prefix=PythonExpression(['"some_prefix" if "1" == "', EnvironmentVariable('SOME_VAR', default_value='0'), '" else ""']),
    )

return launch.LaunchDescription([n])

Anyway, here's my original solution:

I ended up using a custom Substitution. Not sure if that's the cleanest solution, but it worked for me.

class ConditionalText(Substitution):
    def __init__(self, text, condition):
        self.text = text
        self.condition = condition

    def perform(self, context: 'LaunchContext') -> Text:
        if self.condition:
            return self.text
        else:
            return ''

Putting everything together:

def generate_launch_description():
    use_prefix = (EnvironmentVariable('SOME_VAR', default_value='0').perform(LaunchContext()) == '1')
    n = launch_ros.actions.Node(
        package='my_pkg',
        executable='my_node',
        prefix=ConditionalText('some_prefix', use_prefix),
    )

return launch.LaunchDescription([n])