Workshops can already be set to be available only to dwarfs of certain skill levels; in the q workshop menu, it's under P, I think. Workshop profiles, anyways. Allows maximum and minimum skill levels to be set, as well as only permitting certain dwarfs to work at a workshop. It would be possible to implement a very micromanaged and basic version of this using that, and enabling all workshop labors on all dwarfs, though that wouldn't necessarily work very well.
That said, idlers working in workshops is a good idea, as an opt-in thing. That is, off by default, and anyone who wants it will hopefully find it and use it. I probably wouldn't - having a body of ten to twenty dwarfs idling is good for when sudden large tasks show up (bringing items to the depot, for instance), and I would hate to have to go through and kick them all out of workshops.
Although, given that dwarfs are currently rather obsessed with working, and the most effective way to run this would by by setting whatever task is in the workshop to repeat, lots of them would probably get stuck in an endless grind of making blocks or whatever, despite the need for other tasks to be completed. Presumably, mechanics for dealing with that would be part of the implementation of this (tasks being done by an idler would be set to absolute lowest priority, so as soon as something else showed up they would wander off to do that).
Also, it seems like it would make more sense for the end quality to be determined by ... not the average of skills, but some formula involving them. A legendary stonecarver, presented with a horribly half-carved statue, would be able to make something not so horrible of it. Or perhaps just give up and say that it's done ... so having a percentage chance based on the relative levels of all involved, or mental attributes of the higher skilled dwarf, or whatever else, would seem logical.