Here's a prototype using data structures. It has "springs" instead of hard proportions, but it obeys the limits of the slider, which gets closer to an rslider. I think there's a hack to get it to obey hard proportions. Also, I'm just working with one scalar so I'm not sure if I've got class-wide state in the patch (as opposed to "per-scalar" state by storing it as fields in the struct).
Of course this is one of those Pd-style prototypes where it becomes increasingly difficult to do anything beyond the prototype. Try to encapsulate it in an abstraction and you deal with gop resizing issues and state-saving issues. For that matter, try making it resizable and you give up the hard boundaries since quanta values don't accept variables.
-Jonathan