Couldn't you just hit the MixPanel API after the response has been flushed? It means responses are returned as fast as normal and you don't up the complexity of the whole thing by introducing other processes and potentially bottlenecking queues.
Sure, we could echo the response, then call the MixPanel cURL request, but still our API requests would continue blocking waiting for the cURL request to MixPanel to complete. We want API requests to complete as quick as possible. Also, if ever MixPanel's API went down, our API requests would quickly back up, as cURL requests to MixPanel timed out (10 seconds).
And Gearman takes care of re-trying failed tasks for you, too. I just wish there was a good web interface available without having to roll my own (there's https://github.com/brianlmoon/GearmanManager, but it requires php-pcntl rather than the supervisord setup)