Reports within Workfront are simply displaying information that's stored somewhere on one of the WF objects (tasks, projects, issues etc), so I don't think what you're describing would be possible quite that way. That being said, depending on your environment, it is possible to create fields on a custom form that can be limited in visibility.
Two options could be a section on a project-level form that's only visible to those with Manage access to the project. If your environment happens to limit that to only the PM running the project - then that could be a simple solution. If that isn't viable, there are more granular settings available to limit access to the Custom Form itself, and then further limit access/write access to the individual fields on that form.
I haven't specifically tried your use-case with those settings, so it would take some testing to make sure it did what you wanted first.
Katherine