I like the ESL comment: that's cute. But to your broader point I ask, "Why?" If I can comment in an online discussion group or I can post on an online discussion group, why is the relation of my temper any different than the relation of my post?
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.
Actually, I realize I squeezed in let without noticing. Try this...
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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.
My thought process was that the online discussion group is the target or the victim, not the venue, of the loosing. In which case "on", or "at" for some other direct objects (like "arrows") would be the appropriate preposition. As in, I loose my temper (the hounds) on the unfortunate souls of Hacker News.
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.