I swamped a kayak support trip once on an S-rig, and it was 12 days long, in order to let the folks play all they wanted.
Three day GC trips probably translate into going from Diamond down. Problem is, Diamond Creek is mile 225 and by 238, you're completely done with the whitewater -- 232 being the last really interesting rapid in my view. From there to within about five miles of the takeout it's moving water, with only one other small rapid at about 277.
You could get half of a commercial trip in for 4-5 days. But the pace
wouldn't be amenable to doing more then jumping off the S-rig, running the rapid, and climbing back on to ride to the next rapid -- little or no play time. And that would entail a hike either in or out at Phantom Ranch. The outfitter might take your gear all the way, but none of this would come cheap. Particularly if you didn't have a corresponding group of people to take the other half of the trip you weren't going to be on. The outfitter would essentially have to sell you a whole trip (at full trip cost) and then dead-head whichever half you weren't using.
I think Cat would be a very viable option, but that is only one day of whitewater out of 4, more or less. So there's a different kind of drawback. But Cat permits are relatively easy to get, and depending on when you want to go on the Grand, booking might already be a problem -- even if the logistics could be solved.
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