This works but it is hardly obvious. I created an aggregate device in Audio MIDI Setup that contains both DACs. I made the 192kHz DAC the Clock source and set it to 192kHz. I made the 96kHz DAC the output DAC. I made the aggregate the output in Pure Music’s audio setup.
I am not a software guru so I’m only guessing but I think the 192kHz DAC is telling Pure Music it is okay to send the audio aggregate a 192kHz signal and that signal is being sent to the 96kHz DAC, which is capable of playing 192kHz signals if you can get them to it. I have searched the internet and can find no explanation of how audio aggregates actually work, so this is pure guess-work.