I think your plan sounds about right. There should still be plenty of viable yeast/bacteria in there once you're done with it so long as it isn't too high an alcohol, is wasn't filtered, or been treated poorly, which sounds like it hasn't. I've not tried anything this intense yet, but check out the Mad Fermentationist's blog
. He's got a bunch of info on growing up and storing dregs.
Take a look at what you've got once the keg is done. If there's quite a bit of dregs in there, you could even pitch them into a small size batch (1-2 gal?) and ferment it out to see how it tastes, while growing up more in the process!