I don't mind doing the image sequence encoding using ffmpeg. I have a workflow set up using 'make' so it's really easy. I did notice that generating a sequence of PNG files is slightly slower than generating a sequence of TIFF files. 

My scene doesn't have any dynamic hair, but it does have conforming hair. I'll disable that to see if anything changes.

The thread behavior is this:

1. After launch, immediate render an image sequence. This works perfectly. The progress dialog counts through the frames as it should and the UI returns when it's done.

2. Render an image sequence a second time. During this, the progress dialog does not count through the frames, the UI is frozen and I'm watching the spinning beachball. However, in the Finder, I can see the image files being generated. Once the last image file has been created, the application remains locked for about five minutes doing nothing but spinning the beachball. CPU is 100%, a symptom of a spin-lock gone wrong. (It's also curious that Poser only ever uses one core.) After about five minutes, the UI recovers and starts behavior normally.

3. Try to render another sequence. This one behaves as #2 above but never recovers. I've let the application sit for 15 minutes and it never comes back, necessitating a force-quit.

I'm pretty sure this is Mac specific as threading libraries are tightly linked to their underlying OS. Likely the programmers are not as familiar with posix threads as they should be. I noticed they use the "libpth" threading library which acts as another layer of abstraction, but the law of leaky abstractions always wins.