Personally I think when the rules say "can be dispelled anytime" it means after that section of the phase. I.E Before you roll a spell or after the effects from that casting are resolved. As the miscast is part of that spell so to speak you would have to wait till after that has been resolved.

One could also argue as Throne was up at the time of casting that spell has already taken effect. I.E if it was a Flesh to Stone it was cast with Throne up so you would get the extra bonus toughness, as one part of the augment spell has taken effect I can't see the reasoning for it to be allowed to be dispelled before the entire effect is taken.

It reminds me about the Tzeentch Call to glory spell where the champion would attack with his stupidly high initiative then before any attacks back at him it would be dispelled so the 100vp's are not given up. Utter garbage.