"From looking at the Rugers, I think that you'd have a problem with topstrap thickness if you moved a rear sight much further forward."
Daniel, that's quite true on the Vaquero. On the Blackhawks and other adjustable-sight models the main body of the rear sight rests between two "rails" that protect it from movement and at least some "impact adjustments". If you pull the rear sight completely you've got these two "walls" running along either edge of the topstrap into which you could cut dovetails. The sight would then form a "bridge" between the two rails, which isn't great unless you "fill in that gap" with something liquid that can harden. Solder would do, perhaps brazing, or even a super-strong epoxy resin like JB-Weld or the "Magic Sculp" epoxy putty I use sometimes. Sounds crazy, I know, but once the whole "gap" area between the "rails" and under most of the new sight is filled in and smoothed it can be painted black.
And you could adjust the rear sight height to match the stock front sight.
But...would this work?