I can confirm that Retrospect (version 3.0) does indeed copy invisible files as I checked on it today. However, I also found out that version 3.0 (1995) is OS 7.0 onwards only, but I also found a copy of version 2.0 (1993-94) but was unable to find any system information for it.

If it does turn out version 2.0 is compatible with System 6.x you're more than welcome to it, it's the original disks, but no manual, although I do have some Retrospect manuals somewhere. When I get time I'll load it onto one of my machines running system 6 (the fx is at the moment) and see if it works, I'll report back.