To get as close as possible, feel-wise, I'd go with Jon Shuker as he can basically clone your bass with a few measurements. Nino does great work from what I've seen over on TB, but I think he's restricted on neck profiles, etc. and as anybody who's played an RBV will tell you the neck profile is pretty special!
Nortstrand DCs are probably the closest pickup on the market to the RBV's Bill Lawrences, although with the Nordies you have to make the decision whether you'll use them in series or parallel most, as they're wound differently depending on this.
lwtait - in theory getting hold of an RBV and having it refinished might be the simplest option, except they're no longer made in 5 string form and there aren't too many around for sale! Nathan only got this one as I didn't see a need for the B string in my current band.... so what have I just replaced it with? Another 5 string. Yes... I'm a tit... that'll teach me (although the Bongo does indeed ROCK!)