That would be cool, but from what I remember aliens don't exist in the Buffyverse and magic (at least how it exists in the Buffyverse) doesn't exist in the Whoverse.
As Trent said, the possibility of aliens is
acceptable within the Buffyverse, and while Torchwood is undeniably tied to the Whoniverse, it has its own rules which are slightly looser. They've already done at least one episode of Torchwood that directly dealt with the paranormal, so that's that.
And the whole reason for introducing Giles to Torchwood in the first place, is to (re)introduce the audience to the actual "Ripper" character, so anything less than Head openly appearing as Giles/Ripper would be kind of pointless.
Granted, I don't think it could happen, but I still think it would be cool. I've thought so since the very first episode of Torchwood, when they explained the rift, and I thought it sounded very much like the Buffyverse description of Sunnydale's hellmouth, except temporal rather than dimensional.