With the use of the following puntuation keys  ;- )  etc. To indicate
a facial expression to the reader.
Is there an approved set  of keystrokes or characters to indicate
mooning?

Hello kemlo!

As you can imagine, there is no particular body which "approves" terms
to be used on the internet. The only way you could judge something
like this to be approved, is by its actual usage by internet users.

The word used to describe these glyphs is "emoticon" - a concatenation
of "emotion" and "icon" - and if you search for emoticon and mooning,
you'll see a few good suggestions. Having made a rather
eyebrow-raising perusal of some of these, I would like to offer the
following as the best (and perhaps most accurately depicted):

(_!_)

You can see the results of the search I did by going to the link below
- please be aware that in a few cases, the results do contain some
adult themes and images.

://www.google.com/search?q=emoticon+mooning

I hope this answers your question! If anything's unclear, though,
please feel free to request clarification before rating this answer.

All the best,

--seizer-ga

Thanks Seizer 
I had never heard of Emoticon. Your research was just the help I needed
Regards Kemlo