I think the biggest reason why they haven't done it before is that while it would do some damage, it would ultimately work against the Consult's goals.
First, it would be incredibly hard to get the legion to an area where it could really do some damage. They'd have to sneak by Sakarpus. And then get through the Kethol Pass. Given that it's more-or-less the TS gateway to the North, I can't imagine that it isn't carefully watched if not well-defended. It seems like a mass of frenzied Sranc in a confined space would be easy fodder, especially when you add sorcerors to the mix.
Supposing that even if they somehow manage to get past Sakarpus *and* get through the Kethol Pass, it probably wouldn't happen so fast that the other countries couldn't mount a formidable defense. In the long run, it would only serve to unite the disparate Three Seas nations against the Consult. In the PON, the Consult's whole MO was to avoid a united resistance by Man and remaining in the shadows until the No-God could be resurrected.
That all being said, with the majority of the TS military might in the field and chaos throughout the Empire, it wouldn't be a huge surprise if this sort of thing happened in TUC. :-)