Is there any way to constrain to a surface?

Reality Hack

I know you can get a prop to constrain to a character's hand, but I'm wondering if there's any way to constrain a character's hand (or even just a node) to the surface of another character. For instance, if one character were to grab hold of another character's arm. Or are you basically relegated to having to keyframe that entire interaction, repositioning the arm, hand, and fingers for every single frame?

Thanks in advance.