I was always intrigued about Giles as "Ripper", and would most certainly adore this project were it to ever come about.
As a televised miniseries, I think it could really work quite well. I'd like to keep it simply though, without Buffy being a part of it (at least not more than a cameo). As much as I'd love to see SMG Buffy in something new, it wouldn't seem right to have Buffy take up a considerable amount of time on a Giles special.
They could at least give us a Giles spin-off comic that would be canon.