Hmm... Off the top of my head, Kengan has shown Zina (with uncle Ivan), the back of Akemi's head in a bonus with her sister Shion as she reminisces about Hatsumi Sen, a Volume Extra crossover in an eating contest between Ohma and Hibiki, the mexican wrestler mentions being in love with Sakaguchi Mei, and like you've said, Karla's mom Yakusha is in Kengan once or twice.
ISSK shows them a tad more in bonus color pages.
Danberu mentions, and shows them nearly every chapter, though. Sandrovich enjoys writing not-fighting personality fun into his Kengan characters by placing 'em in Danberu very regularly.