The idea was to send 100 million outputs per BTC to different addresses of course.
Which is no different than sending 100 million outputs to the same address. The number of unique addresses is irrelivent.
Still even before IsDust this attack wouldn't work. Low priority tx are not relayed unless they pay a fee and that fee would cost you 0.1 mBTC per KB. So sure you can add 1 GB to the blockchain, no problem. It just will cost you 100 BTC minimum in fees per GB plus the cost of the value of the tx themselves. Also given that miners priority tx by fee amount and/or priority your low priority tx would simply gain the unused space in the blocks and thus it might take months or years to bloat the blockchain an extra GB (and this cost you a huge sum).
So IsDust is simply a secondary line of defense (it actually is intended to prevent another type of resource hog), the attack wasn't viable even before IsDust.
