AF-Assist options for pro bodies
I just tested my SB-800 it can do exactly what I think you are asking for. There is an option in the menu on the flash to turn "Fire" on or off. If you select "off" in the back LCD it then displays "AF-ILL ONLY" and indeed, only the red AF illumination light is used, and the flash does not fire. I would thus fully expect the SB-900 and SB-910 also offer this feature. I have no idea about the smaller flash units.
Now, one other point that may be important to you. None of the AF-Illumination lights on Nikon's flash units (at least SB-800 to SB-910) work in AF-C mode, the AF-illumination light is simply turned off in AF-C, it only operates in AF-S, which is a huge disadvantage which might impact your shooting needs too. As far as I'm aware, there's no solution at all for AF-C assist. If there is, and someone can call it out that would be awesome.
Gallery at http://www.rolandwooster.com