The Mirage never existed. This appeared in a number of Amiga magazines at the time, although god knows where the rumour came from originally. The instructions make no sense and make reference to an Imperial flagship - which also doesn't exist - and the edge of the galaxy (err which edge?) Nevertheless, we still played for ages living in hope that we could find it.
The Thargoid ships are coded into Frontier, I think. Certainly the ship model used to show up in some of the bases you were sent to attack or photograph and there were hacked save games that let you fly it and other ships in the game such as the Long Range Cruiser and Lynx Bulk Carrier that were not otherwise flyable. (I believe the above screenshot is an example of this) I've never seen a Thargoid ship actually in flight though, and I've clocked up a lot of time in Frontier over the years.