Thanks Goony, it's a one-off, scratch-built. I used it in ratio mode, 2:1 (bounced:direct). As long as I remembered to compensate for very light or very dark subjects, this setup gave me lighting that often looked almost as good as studio lighting(to most people) in a handheld package. The TTL-OTF system on the 7000i/8000i had very good fill/backlight performance and this rig put it over the top for handheld event-type photography IMHO. I think one of the reasons it worked so well is the relatively large/powerful forward-facing 3200i, especially compared to the Metz mentioned above, plus the finer control. Whenever the bounce wasn't quite enough(high/dark ceiling) the 3200i took up the slack; worst case I would get a shot that looked a bit more like typical on-camera flash than the other shots, but still better than with a single forward-facing flash.
These bodies had locator pins in the bottom plate that I utilized to stabilize the bracket:
The threaded hole served dual-duty; I could mount the rig on a tripod if desired but mostly it provided a storage spot to keep the screw from getting lost when the bracket was detached from the camera.
I've done a little reading up and it seems the F42 is the only flash smaller than the F58 that can be controlled in ratio mode, though the very confusing F58 owner manual and Gary Freidman's book seem to disagree on this.
Unless something more like the 3200i shown above is announced at PMA I will probably get the F42 and start figuring something out. The a900 only has a single hole on the base plate that can be used by a locating pin, so I'm thinking along the lines of a second flash bracket that would attach to the left side dovetail of an RRS L-plate(which is on my list to get soon anyway). The basic design layout is going to have to be re-thought if I'm going to use an F42, since it's so much taller -- maybe have the bracket swoop down from the camera body to a handle that would then mount the F42. It may not be possible due to the way the F58 shifts for portrait shooting, though it may be that simply moving the F42 forward would get it out of the way.
I may rent an F42 and see if I can get the behavior that I want before buying another flashgun. I have 6 iISO flashes already not counting the ring flashes, though the i series only work in manual mode, wired or via flash zebra. That's plenty for just about any amateur shooting I can forsee doing in the near future, but if I need to spring for another flash to duplicate the above rig I'll do it. I know it can be done with my second F58 but it's just too big.