Calumnies and coprophagist aren't particularly obscure. I've come across both.

I think you have to discriminate between slightly obscure or archaic words that anyone familiar with a reasonable range of the literary canon would know, and truly uninteresting words that even a highly educated and well-read person wouldn't know.

There are better corpuses than Wikipedia that could be used for this purpose, like the British National Corpus

http://www.natcorp.ox.ac.uk/