What? Covering a large part of your body doesn't necessarily mean you can do a full-body Koka. I mean, if you wanna take that as to mean he can, do you, but that panel is not solid proof. Also, the legs are naturally stronger than the arms, so if he was planning on freeing himself, it'd make sense if he used both. There's also the fact that Sai uses a kick-based fighting style, so his legs, like Sanji's, should be stronger than his arms. It'd make more sense if Sai had used Koka on his leg.Well if you use logic then the rest of his body is also covered in armament, considering he would die or lose body parts if he didn't cover the areas which are being impaled. Regardless of one leg, he can still use full body armament. He simply didn't cover that leg because it wont help him get out of that attack. He would have no way of getting out with just one leg. That's why he coated the visible arm and not that portion of the leg.
Also its possible that the leg that is covered with his pants that are visible in that panel are being covered with at least some armament by his mid section. Or else it would get smashed and probably chopped off.
Anyways, the fact that his leg has no Koka means that that panel does not prove he can use a full-body Koka.