I think I've been solving similar issue in the foreman plugin.

The nature of the problem lies in the fact that you're listing a resource different from the one you need searchables for. I solved it by creating a dedicated command class:
https://github.com/theforeman/hammer-cli-foreman/blob/master/lib/hammer_cli_foreman/commands.rb#L228
Useage example can be found in:
https://github.com/theforeman/hammer-cli-foreman/blob/master/lib/hammer_cli_foreman/host.rb#L265

Would it help in this case?
Something like...
```ruby
class TasksCommand < HammerCLIForeman::AssociatedResourceListCommand
      resource :tasks, :index
      parent_resource :systems
      # ...
end
```