In the recent Qingdao Grand Prix, Italy's Manuel Parlati did a very clever and opportunistic leg grab kosoto that is perfectly legal.
He started off by doing a drop ouchi-gari on his opponent, Abylaikahn Zhubanazar (KAZ). The ouchi completely misses but Parlati immediately follows up with sumi-gaeshi. Zhubanazar kills it dead. Parlati turns over onto his front and Zhubanazar casually walks over him.
The thing is, Parlati's left hand still has a grip on Zhubanazar's right sleeve. As long as tori's hand is still holding onto uke's sleeve, any touching of the legs is permissible.
By the time Zhubanazar realizes what is going on, it's too late. Parlati pulls up on Zhubanazar's right arm, which causes his left leg to be lifted up. Parlati then proceeds to sweep away Zhubanazar's right leg.
This is perfectly legal.
