This is an extremely niche feature... And no, it is not hard-coded into the viewer.
You could, however, use
Lua scripting to implement it yourself. See the radar related callbacks, and specifically the OnRadar() one: associated with some minimal code to store the new incoming avatars together with a GetFrameTimeSeconds() time stamp in a Lua table, you could set up a Lua side bar button or chat trigger to display that table entries in the proper order (with either a "poor man" method by printing the names in chat, or with a luxury interface using a custom Lua floater with a sorted list)...