Interesting thread.
I actually wrote a humorous blog about this fenomenon.
The first rule was not to wave to cars with the hood up as you can not see if they wave back or not.

The second rule was that all other sports cars have to wave first if they meet a Morgan.

There were quite a lot of other rules as well, but can not remember at the moment.
However tody on mine and my wifes 110 mile tour along the coast today with actuallu met two Morgans.

Both with the top down of course and both waved.

In Sweden we usually stop and have a chat when we meet Morgans, but today it there was no good place to stop.

We also met a couple of late Jaguars with the top down, but we did not wave and neither did they. A nice MGTC on the road waved frenetically and showed good manner and respect for our Morgan.

A lot of Miatas and Alfa Romeos were also on the roads today, but it does not seem that these people are as much real enthusiasts as people with British sports cars.

But, i hope I am wrong.
Maybe they will learn!