It's not a question of, "Is Orianna's kit ok for this?" It's really a question of "Can she do this better than other champions who do it?" I just don't see it being better than Kog'Maw or Teemo or someone with this build.
That's not to say it won't work, I mean you can do this with just about anyone and do well if you have a good game or get lucky or something, and clearly Orianna's better than like a Veigar or someone, but I don't think Orianna's ideal.
Orianna vs Kog'Maw, I'd vote Kog'Maw. He gets that increased range and the health scaling.
Orianna vs Teemo, we can make the comparison pretty easily.
Maxed Toxic Shot does 45 on hit and 30 dps on the poison. He can max this before Orianna's passive maxed out, though.
Maxed Clockwork Windup does 30 on hit, but 115% of (115% of (115% of 30) is 45 damage as well, so when she gets the multiple hits up, it's roughly equal.
Orianna's has slightly better AP scaling by .06
So you're trading out Teemo Shrooms and Blinding Shot for Shield, AOE slow, and Orianna's Ultimate.
Teemo has a slightly better early game, but Orianna might bring a stronger late game from her spell damage, shields, and soft CC.
It's basically a fair tradeoff, I could see legitimate arguments for either one of them.
So Orianna > Kog'Maw, Orianna arguably equivalent with Teemo.