Knowing Smogon, who end up banning everything, I don't think Grassy Terrain itself would be banned, or even Nature Power, or even the exact combination of "A Shaymin who is on the same team as another Pokemon that can summon Grassy Terrain and knows the move Nature Power". 60% isn't much compared to the 100% accuracy Spore has, which several Pokemon can learn, or even a Compound Eyes Pokemon that uses Sleep Powder (97.5% accuracy), or just sleep powder itself (75% accuracy).
Not broken at all, and even possibly a little gimmicky. I don't see Smogon banning it anytime soon even after Gen VII hits, since that means they'd likely have to ban all sleep moves. Which is... kind of silly and removes the entire point of the Sleep Clause in general.
As for official tournaments, Shaymin's already banned so that entire point is moot.