>I'll probably also be using a Tyvek ground cloth, is that a reasonable choice?
I highly recommend the GossamerGear Polycryo ground sheet. It's tough, completely waterproof, and is light (1.3-1.5 oz.) compared to Tyvek (about 5.5 oz.).
>I just saw the Gatewood Cape poncho/tarp on the site and thought I might try a cape+pole+Oware bivy combination as shelter.
You won't need to use a bivy with the Cape. If it's cold, windy or rainy, just stake it down to the ground. Then there isn't any exposure around the back and sides, and several inches of gap (for ventilation) on the vestibule, which is several feet from the sleeping area. It might get some condensation (mine hasn't yet) but that can be wiped down. I tied an overhand knot in the middle of the 6" stake-out loops and use the 3" loop length rather than staking it completely down when I want it closer to the ground. Otherwise, the 6" stake-out loops allow it to be quite a ways off the ground and provide plenty of ventilation, while still keeping non-blown rain out. You can prop the hood open (I clip the front of the hood to the front guy line) and get a real chimney-effect going.
>Has anyone seen/tried the GW Cape? If so, how durable does it seem regarding fabric weight and stitching?
I've found mine to be very sturdy. In shelter mode I really stretch it out taut, and when hiking the other day I snagged it at least a half-dozen times on unyielding broken branches while crossing downed trees, and it just slipped off without any damage. I should be more careful, but so far it's showing no sign of my abuse.
>On one trip, a down bag lost a lot of loft just from the ambient moisture it picked up from the air.
That's a good thing to keep in mind in the Olympics. It might not get very cold, but surviving a week of drizzle is going to require careful attention to keeping a down bag dry both in the shelter and in the pack.