| Hi - just seen your post! You've probably built that bass by now, but for what it's worth I have a set of these Hipshot monorails on my Indie IB604 Custom, and I can't speak highly enough of them. The attack and sustain are remarkable - natural swamp ash body with oil finish, so that helps of course - they stay exactly where they're set in terms of intonation and action, and changing strings couldn't be simpler (the saddles don't try and come away with the strings like some monorails!): they just drop in through their slots, and then lock under string tension. (You can also install them as thru-body units if you like - each one comes with an all-in-one string tube/ferrule/bolt thingy.)
Like you I've never seen them on a J Bass (mine's more like a cross between a Soundgear and a Warwick - 2HB, active electronics) but I can't see why not, even if the conventional path would be a Hipshot A or a Badass 2. | 
