It's almost like Sion's rework would be a good time to implement a change like this if it was deemed necessary. How crazy!
There are multiple single target burst mages. They all manage to function without Veigar's passive, so I don't really think it's 100% necessary at all.
The meta has been screwing over Nasus since 2010. He has been screwed over by pretty much every single meta that has ever existed. I think we can stop using that as an excuse now.
Nobody bursts as well as Veigar. Nobody. And he trades fucking everything for it. He needs that scaling badly. The only thing he has going for him is that he has what's basically an ult that's useful in teamfights on his E.
You're acting as if Sion's rework is coming any time soon in the next year. Riot hasn't even begun to look into him. Karma, Heimerdinger, and Viktor are coming first as far as I know, and if the previous reworks are any indication, those could easily fill until halfway into 2014.
Nasus hasn't been getting fucked over by the meta since 2010. The exact day he got screwed over was the 22nd of November in 2011, when S2 masteries went live. He had trouble previously, especially by metagolem, but that was because of standard endgame supercarry early-game shittiness, not because he couldn't do anything. He's had a much better time of it since S2 ended. Honestly, he can do with some mana cost reductions on his E, slight cd reduction on his R, and he'd be just fine. His passive and his Q actually are quite strong now.
I'm extremely, extremely hesitant how powerful that hook Q will be. 1.5 seconds IS a long time, but it's uncleansible. Once you get grabbed by it there's nothing you can do about it. The range on it is, if I recall correctly, 150 range longer than blitz's q, and it works much the same. There's going to be a lot of wallgrabs and the cooldown is quite a bit lower than Blitz's.