There's a yahoo group that may be of help to you - njb3tapers.
That being said, I know of quite a few people that are running Mics->batt box -> JB3. I tried that for a while, then decided to get an outboard preamp+a/d. The AD-20, as suggested by zhianosatch. It's a great unit, definitely stealthable. The only problem is that you'd be going mics->batt box->ad-20->jb3. A lot of little pieces. I, personally, found that frustrating. But I also wasn't stealthing and trying to sneak gear in.
I'm not sure what kind of mics you are running, but Sound professionals does make a battery box with a preamp built-in, and they also have a preamp that provides plug-in power. I actually ran their version of at853->analog preamp(w/ plug-in power)->jb3 with good results. I found that I wanted bass rolloff, though, which meant ditching that preamp and getting the AD-20. I believe it's pretty well accepted that the AD-20 has a better A/D converter than the one built-in the JB3, but the internal one really isn't that bad.
Yeah, sorry for the random mass of information. Hopefully some of it helped.
J