I would have thought that even in the examples given, the word 'viscera' is still being used as a plural. As a medico I use the singular 'viscus' not infrequently - e.g.. 'gas under the diaphragm is a sign of perforation of a hollow viscus'!

Furthermore, there is a difference between truce and insignia on the one hand, and graffiti and viscera on the other, in that the first two are the only ones that are truly plurals that have come to represent a singular item. (You can have 'a truce' and 'an insignia' but not 'a graffiti' or 'a viscera'.)

'Ephemera' would be another in the same category as viscera I would think....