Sanji's fighting style is using his leg and his epithet is also called Kuroashi no Sanji / Black Leg Sanji. He learnt this style of fighting from Aka-Ashi no Zeff / Red-Leg Zeff.
As far as I remember, Okamas also use (at least) their legs to fight, like Bentham and Emporio Ivankov.
My questions are :
1. The Longleg Tribe uses their legs to fight.
The incredible long legs of the Longleg tribe endows them with not only height but incredible leg strength as well, and those that are hit by the legs of a Longleg fighter consider them to be like metal whips.
As acknowledged by fighters in the Corrida Colosseum, the legs are the most important part of the tribe's fighting form. Therefore, aiming for them seems to be their most vulnerable weakness. However, due to the speed a trained Longleg fighter can move their legs at, landing just a single blow is not always possible to do
This includes:
• Blue Gilly : You can see him use his legs for fighting in this video. He is also a practitioner of Jao Kun Do.
Jao Kun Do is a close range attack-based fighting style which involves leg motions so swift that foes cannot predict where a kick will land.
The only known practitioner of this art, Blue Gilly, has leg movements so rapid that they create afterimages that confuse opponents. He can then attack with force comparable to that of a steel whip.
• Baron Tamago : We haven't yet seen him fight in the anime but it's implied that his key fighting strength lies in his legs. As One Piece Wiki says:
As a member of the Longleg Tribe, his long legs are considered to be the main point of his combat strength
2. Inuppe, the zombie penguin who had Sanji's shadow uses the Black Leg Style fighting techniques as well.
In the Thriller Bark Arc, Inuppe, the zombie who had Sanji's shadow also used the Black Leg fighting style due to the Kage Kage no Mi's powers.
3. Vergo can be considered a strong leg fighter as well, although he does far from only using his legs. You can see his fight here with Sanji where he uses his legs for fighting for most of the part.
In general there are plenty martial artists in One Piece, in particular there are some styles which include also leg techniques.
For example Rokushiki includes the following techniques
- The Geppo allows the users to actually jump off the air itself, allowing them to stay in the air for much longer than usual. CP9 members can use this technique to cross great distances without ever touching the ground, or set themselves up for swift, aerial attacks.
- The Rankyaku is a powerful projectile technique, in which the users start by kicking at very high speeds and strength, sending out a sharp compressed air blade that can slice objects and greatly damage a human body.2 However, it has been shown that almost any length of appendage (such as a tail or neck) at high velocities are also capable of this move.
- The Soru allows the users to move at extremely high speeds in order to avoid attacks, as well as to attack at higher speeds and with greater power. It was revealed that the principle of this move was to kick off the ground at least ten times in the blink of an eye.
Also Hassoken can be adapted to leg-combat, in particular Sai's techniques.
As you mentioned some Okama Kenpo users are specialized in leg combat, e.g. Bentham.
Moreover you have Blue Gilly (style: Jao Kun Do) from Longleg Tribe, who was shown fighting in Dressrosa arena.
Another leg-combatant is Dellinger, from Donquixote family.
If you consider also not-canon character you should take into account also Hotdog (who appeared in the third movie)
Dellinger is one such character that uses kicking as his fighting style. As from one-piece wikia.
Dellinger is a combatant and an officer of the Donquixote Pirates' Diamante Army. He is a hybrid between a Human and a Fighting Fish Fishman.
A human-fishman hybrid, Dellinger was born physically stronger than the average human, going by his ability to lift cannonballs at two years of age without problems and the fact that full-blooded fishmen are supposedly ten times stronger than humans, but it's unknown if he's as strong as an average fishman in relation to a human
Dellinger is extremely proficient in kicking, seen when he easily knocks out Blue Gilly, a powerful martial artist known for his own strong kicking, with a single kick while leaving no time for the latter to react. His kicking strength is further highlighted by the fact he is able to kick and move Machvise, who at the time weighs 10 tons thanks to his Devil Fruit, though Dellinger shouts in pain a result of the kick.
Pistol High Heel (ピストルハイヒール Pisutoru Haihīru?): Dellinger rushes towards to his enemy and kicking him with force while wearing high heeled shoes. It is powerful enough to hurt even super-weighted humans. It's first used accidentally on Machvise.
Danto High Heel (断頭ハイヒール Dantō Hai Hīru?, literally meaning "Decapitation High Heel"):- Dellinger charges at his enemy and unleashes a hard kick to the neck, potentially strong enough to cut an opponent's head off given its name. It's first used against Blue Gilly and is powerful enough to take him out in one blow.
