Q: The pural opi for opus is a joke, right? https://en.wiktionary.org/wiki/opus attests that some people in the classical music world use "opi" as a plural for "opus."  I think this is just a joke, giving a pseudo-learned false-Latin form for the plural.  But I want to make sure.  
Can anyone here who is familiar with the usage at first hand tell me whether it is generally meant as a joke?

A: (Dredging up my schoolgirl Latin) Many Latin nouns ending in -us are second declension, with plural -i. However, opus happens to be a third declension noun, and its plural is opera. See latindictionary.wikidot.com/noun:opus 
As others have pointed out, octopus and platypus are derived from Greek, so they don't have plurals in -i either. 

A: Yes, it is an in-joke, which I can confirm as a former instrument player in orchestras.
It's not confined to the arts world though. I have heard people jokingly pluralise other words with -i that don't even end with -us. It's probably more common among those who learnt Latin at school (as I did).
Allied is the use of 'platypi' colloquially but in this case the word platypus was neologized from Greek roots.
