Do we know if

Jareth, Leonine Titan
Arcanis, the Omnipotent
Visara the Dreadful
Rorix Bladewing
Silvos, Rogue Elemental

Made any cameos on the Onslaught novels or in any supplementary storyline articles (Savor the Flavor?)?

As far as I know, these blurbs from Rei Nakazawa are all we ever got (Apart from the fan fiction I wrote back in 2012, but that doesn't really count): http://magic.wizards.com/en/articles/ar ... 2003-01-16

They were bottom up legends, and didn't get much love otherwise. I feel like Rorix got an official story somewhere, though. But I'm usually wrong about these things.

All I know about any of them is that Rorix dies in the pit and then gets reanimated.

Arcanis was featured in the Battlegrounds game, where it was stated he got his power from visiting the ruins of Tolaria or something like that. The Battlegrounds game was nonsense canon-wise (In the campaign you fought against Maraxus, Akroma and Mishra), but the bit about him visiting Tolaria's ruins could be true, I guess.

But the whole point about Arcanis is that he's mysterious! Explaining him ruins him!

And yes, Rorix died and was necromanced back as Bladewing the Risen. But I believe that was another case of bottom up design, so we get no story there either. Although it is implied that after being zombified he got the power to raise zombie minions of his own (see Bladewing's Thrall). But that's pretty much it.

Tarox is honestly more interesting, what with all the fratricide.

I liked the Future Sight "Grandeur" legends. I was particularly fond of Korlash, Heir to Blackblade. I had a really fun casual deck centered around discarding and returning Korashes to my hand like crazy.