One would loose one's temper \on\ a discussion group, not \in\ it. I enjoy the perspectives of the ESL folks on HN, though, so I don't want to see any filtering by grammar nitpicking.
The OED is useful here: it literally defines in as equal to on in the second definition of the prepositional form of 'in'. There's an extended history of the relation between 'in' and 'on', which includes the Latinate source of the Saxon term, its evolution in Old and Middle English, and its use today. Suffice it to say, the distinction is not a trivial one, and 'in' and 'on' are often interchangeable.
You can simply say: "He let loose his temper." so...
It is poor form to sleep in a meeting. It is poor form to scream in a meeting. It is poor form to loose your temper in a meeting.
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"on" sounds much more natural to me, and I think that's enough for me. Idiom counts in a language, too. In and on might seem technically interchangeable, but (sometimes) if it sounds wrong, it's wrong.
You can say, "I lose my temper." So like you can say "It is poor form to sleep in a meeting.", you can also say, "It is poor form to lose your temper in a meeting."
As someone else commented, you definitely can say, "I let loose my temper." So, "It is poor form to let loose your temper in a meeting." is fine.
But, can you say, "I loose my temper."? I think you can, but it is very very unnatural.