I wonder what it's like to knock one off in 'zero g'

Must have happened, on the ISS.  Impossible for a healthy chap to go months without any sort of release.  In fact, if you didn't, I believe it would happen involuntarily after a while anyway.  Much better to have a 'controlled explosion', if you will.  Maybe there's a device they do it into.  They have to be careful with urination and defecation so it makes sense.