There was always a good chance Hieshyn would know all the poems, I think it mostly like Beet just didn't get one for reasons which even if sinister are likely non-obvious, and just bad luck it was that poem. Whichever, odds still indicate Hieshyn as the most likely author of that poem, however Hieshyn has never (beyond posting the word for poem confirmation) used the words Mistborn, Brandon, or Sanderson either here or on the books board, so he may not have even read the series for all I know and I'm sure the author of that poem must have, with the alternate being they are very scummy and their RM talked about butchering nobles. Either way that player is dead if we figure out who wrote it... unfortunately you can pretty much strike any chance of word analysis helping since if there's one thing that will obliterate writing style it's trying for some 5-7-5 setup.

Here are people who have explicitly said they haven't read the series, assume incomplete:
White Flame
Fanatic-Templar*

Definitely have read:
Isaac*
Kronin
Beet*
Fox
Stephen

*Confirmed poem writer.

Unknown but unlikely
?

Unknown
Amys
Hieshyn
Praziquantel
Yunalesca

Note that all of these people in the last group have posted to the books board at least once and never used the word mistborn, this doesn't necessarily mean much. John's got about 200 book posting whereas Amys and Hieshyn each have 2, and Praz and Lucy about 20, but I do think it improbable, considering how many post he has, that John wouldn't have shown up in any mistborn thread on the books board, it being one of the more common series to discuss, and he never used 'mistborn, 'brandon', or sanderson'. As to the others, there is simply no way to know, and they could easily have started the series. The others simply don't have a trail big enough to guess off of, especially Hieshyn and Amys.

Still, Kronin, Fox, and Stephen, as non-claimants to other poems and known Mistborn readers, would be prime candidates for that poem, alternatively White Flame, ?, beet, FT, and myself pretty much can't be. I encourage anyone on that list of unknowns (Praz, Yuna, Hieshyn, and Amys) to poke through their own posts to see if they have any disclaimers lying about anyway to show they haven't read it and others to poke around for anyone who may have confessed familiarity or an intention to read it, there's a very 'early book 1' flavor about the poem, commenting about how skaa can be allomancers, so its entirely possibly someone was working their way through.

Really hard to say though, for instance Hieshyn when he signed up for the game signed in by saying Yes to the game .... no to the poetry whereas Stephen signed up with Sounds Fantastic... Warning, you may be subjected to my Edgar Allen Poe meets beat poet word stylings. and has read Mistborn and did join Hieshyn in objecting to discussing the poems even though, in his defense, he did so while giving his own data.

So there are my thoughts for the moment, I'll have to leave it to you guys to follow up on them as I'm RBIRL-ish for another day or two.