Most ABGs are really limited in the ability to mimic the sound of an upright IMO regardless of the strings you use.
If you really want to get close you should consider a Rob Allen MB-2 bass.
I played one of those for about three years in a jazz trio and everyone thought it sounded a lot like an upright, including me.
I came across a custom fretless 5 with lightwave optical pickups about a year ago and I'm currently using that for the gigs where the "upright" sound is desired.
The optical pickups give it a sound like a doublebass with steel flatwounds where as the Rob Allen sounded like an upright with gut strings. There was a subtle difference between the two but these days I prefer the sound of the lightwave pickups
There are some solutions out there that will get you close to an upright sounds however there really is nothing that sounds "just like" one except for the real thing.