吃豆腐 (Chī dòufu): You're right, "Eating someone's tofu" can be physical actions (copping a feel) or biting someone's style. But I think this is as close to what you're looking for in English.
These also work:
Description: 调情时抚摸 (Tiáoqíng shí fǔmō) Touching someone while flirting
耍流氓 (Shuǎ liú máng): Hoodlum. Or what someone may call someone who copped a feel (not so much the act of copping a feel).
咸猪手 (Xián zhūshǒu): Someone dirty who gropes at people, tries to "cop a feel". (lit: Salty Pig Hand). Ex: 请你的咸猪手跟我的朋友保持距离 (Qǐng nǐ de xián zhūshǒu gēn wǒ de péngyǒu bǎochí jùlí) Please keep your salty pig hands away from my friend
动手动脚 (Dòngshǒu dòng jiǎo): be "handsy" with someone (lit: move hands move feet)