Interesting thread. Here's my take. I've just gone through the same quest. I actually own and play a DB and am having ergonomic issues with vertical bass so I've been exploring a BG substitute. To my ears listening to all the samples online and what I've been able to audition the Rob Allen mouse is the closest you can get. Sort of defies the laws of physics that a 30 inch scale BG best mimics a 41 inch scale 3/4 upright.
Now to the larger question: do you really want to "sound exactly like an upright." To my ears, the DB sounds the most DB when its not amplified at all or slightly helped. Frankly, with most drummers I jam with, this isn't possible. Therefore, I end up cranking the amp and get into piezo "honk."
In moving back to BG for Jazz, I've been using a Warmoth J fret-less I cobbled together and have noticed that not only is it more comfortable to play but is more defined and is easier to hear in the band mix. No, I can't get that DB "DOM" envelope but with flat wounds, high action, right hand technique and position, and pickup selection, it is very "natural" sounding. But in many ways it has better definition than DB.
On a final note, let me say that every time I show up to a gig or a new jam with the BG, I see the look of disappointment in the eyes of the other players but more importantly at a gig, the audience. There is a great perception in Jazz these days that if you're not playing DB, its not "real" Jazz. In fact, I'm convinced that I get called for gigs because I do play DB. The audience doesn't know anything about "band mix" and really doesn't care if my shoulder and back aches after playing for two hours. It is after all show business. They hire a live band because they want the "look" of a Jazz combo as well as the sound. Otherwise, they'd just put in a CD.
So for my gig this coming up weekend, I'm doing my stretches working on my arm positioning and sucking it up and taking the doghouse.
