...you may want to consider it rare.
Well, I certainly won't consider it common, EF, as I can't find it in any standard dictionary. However, as you note, the word is in some use, and definitions can be found in various sources, so perhaps we should allow it - as a rare word.
It's
defined in Mosby's Medical Dictionary:
an individual who maintains the hospital laboratory or equipment and facilities. The morgue diener may also assist the pathologist in performing autopsies.
Wikipedia says:
The word Diener is German for servant. In English, it is generally used to describe the person, in the morgue, responsible for handling, moving, and cleaning the corpse (though, at some institutions dieners perform the entire dissection at autopsy). It is derived from the German word Leichendiener, which literally means corpse servant.
Wikipedia also tells us of the word being used as the title of a TV episode and in the name of a poem. However, there is no entry for it, as an English word, in Wiktionary.
The
Wordcraft website gives a definition of
diener, noting that "So few dictionaries have this word – it’s not even in OED". This site gives three citations from popular fiction.
In fact, quite a few fiction writers have used the word
diener - it's not hard to track down examples using Google Book Search. Perhaps this is a reflection of the grisly subject matter of much recent mass-market writing. Anyhow, it is this usage in fiction that persuades me the word is not too obscure for our list.
