Hey Doug,
We just keep bumping into each other...
What exactly are you trying to do with the exposure? Multiple exposures, or one combined exposure for a clean shot?
I was forwarded your post by a team member, and Steve also linked me over to the Multiple Shot & Long Exposure box that Kapture Group makes to fill just this purpose. The issue is keeping the back exposing while the shutter itself closes. Basically, you hit the wakeup signal, start exposing, then shoot the camera. You can see it on this page here:
http://www.kapturegroup.com/phase/phase.html#multivia the page:
The Kapture Group Multi Shot & Long Exposure control unit is designed to allow multiple exposures on a single digital capture and allow you to make long exposures for applications such as digital light painting or multi pop flash shots.
This device is available with the P/IQ series multi-pin or the older H sub-mini connector. It provides the wake up signal and controls the duration of the sync signal to the digital back. This permits the user to layer multiple flash or multiple exposures onto a single digital capture.
Let me know if you're interested!