[DouglasCole]

If you check Force Posture Change, there's a bit about the attacker changing position voluntarily, going from standing to prone is +4, standing to crawling/sitting is +2 (kneeling to prone is also +2), etc in bonuses to the contest. You could allow the defender to choose from this as well, which would get you close to the sprawl (you could also cap the max bonus at +3 instead of +4, which would duplicate the MA sprawl; I was in love with doubling bonuses at the time, so +1, +2, +4; the +4 also felt kinda all-out to me, so there it was).

But there's also a note in TG called "Impossible Positions," and sometimes you just can't maintain a grapple because of physiology.