This really, really depends on your interpretation of how metamagic affects the casting time, and what the definition of "greater than one action" is.
As far as I'm concerned, the casting time of a Still Magic Missile cast by a sorcerer and by a wizard is identical. The sorcerer just needs a full-round action to do it. The stat block of the spell is unaffected.
Other people prefer to view it as the casting time of the spell actually increasing.
And then there is the debate as to whether "greater than one action" includes "one standard action" and "one full-round action", or just the standard action.
My answer? Yes, a sorcerer can cast a Still Spell in a grapple, providing he has any material components in hand.