Ah, it appears that it's actually possible to run a Windows command line from within a PoserPython script, using shell=True and import subprocess. It appears it should look something like this?

cmd = "magick mogrify -layers 'optimize' -fuzz 7% poser.gif" 
run_cmd = subprocess.call(cmd, shell=True)

Although I'm not sure if that would work on its own for Poser 11 and Python 2.7. I have another example that suggests square brackets are needed for 2.7:

subprocess.call(["ls", "-l"])

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Learn the Secrets of Poser 11 and Line-art Filters.