(I'm showing my age. That title makes Harry Chapin's harrowing song "Sniper" go through my head--but is that important? Not now...)
I don't think it's going to be any time soon that well-endowed mesh avatars (or any of a bunch of other mesh avatars) will work with sliders such as breast size, based on experimentation. The slider behavior is, I think, too tied to the specifics of the stock avatar and the limits its designer(s) imposed on it. The way the avatar is modified for different non-skeletal slider settings will have to be a function of the avatar.
To have a chance of getting what we want, we have to ask for something that has what we want as a subset and will benefit a lot of people. How many mesh avatar users are out there? Unfortunately, limitations on such avatars will mean the answer is "fewer than there might be", which in turn means fewer who would benefit from a proposed change, the usual loop that works against change.
UPDATE: I went ahead and asked about this issue and another in passing on the SL Forums (and thanks to the moderator who moved it to the appropriate forum!), and the responses have started. I am glad that people who know a lot more about these issues than I do are commenting. The thread is over in the mesh forum.