I've been on BB for close to 10 years and I have never once heard anything like this. I've never heard about an alternate idea for S6, nor have I heard anything about Lucas being involved, and, as much as I hate S6, this idea kinda sucks, to be perfectly honest.
The whole idea about S6 was 'Life is the Big Bad', and I feel that, if Buffy was rescued from a hell dimension, that idea would have been lost pretty early on. Real life, in S6, was shown to be Buffy's own personal hell, because of her being ripped from heaven. If she was rescued from a hell dimension, that whole metaphor goes out the window.
I feel like there would be too much that would have to be changed for this idea to even begin to be plausible. Buffy and Spike, for example, would have either had to have been scrapped completely, or Spike's character would have had to have been drastically changed. Buffy felt herself to be at rock bottom, so she went to Spike to make herself feel something. That idea wouldn't work in the scenario proposed.
And I don't know if Joss would want to bring in someone like Lucas to essentially take over his baby. Even just looking at their body of work, Lucas and Joss have vastly different ideas. I'm not sure if they would be compatible enough to work together to produce even a season of television.