The question still comes up as just where in the buffyverse time frame will this movie take place. About the only "open" times available would be the year or so between the destruction of Sunnydale and the beginning of the "No Future For You" arc or somewhere later than that, otherwise it's gonna go off canon, not that that would cause Joss a moment's pause. Tony's got the same problem James does: age. He can't play Giles BEFORE he met Buffy, he's just too old. That means that, unless we have a old Giles telling a group of young Slayers how it was in "the good ol' days," then slipping into flashback mode, it's gonna be hard to pull it off.
(...unless, of course, "Ripper" is about the OTHER Giles, the one from the "Wish,Doppelgang" universe...)