there's this big bank in Brazil that requires customers to install pseudosecurity spyware when they're running browsers that identify any of the operating systems for which they have spyware binaries, but that would disregard the "extra security" for those who ran e.g. FreeBSD.

I figured I'd try to change the UserAgent to say GNU as the operating system name, and that worked too.

You know what did NOT work with that change?   a BigBlueButton instance maintained by a major Free Software and GNU-supporting organization :-/