> I'm not aware of any natlangs which do that, but that's no proof they
don't
> exist. I do know that I was brainstorming a year ago with a friend re:
> mechanisms not found in natlangs, and one of the few we came up with was
> reversing the spelling of a word to create the inverse.
Since no one else picked up on this chance to quote auxlang history,
I'll take the ball and run with it. Solresol did this (the 19th century
interlanguage based on musical notes and their names). Interestingly
enough, the name Solresol itself it its own antonym. :)
Jake