1) The Condor Trail. Put this together with the Bigfoot Trail and you've got a whole new way to walk the length of California that isn't the PCT.
2) Put a USB battery pack in one of your later dropbags at a 100 miler, just before where you would expect your GPS watch to die. Also the cord that connects your GPS watch to a USB charging station. Also a cheap-o regular watch. Then, when your Garmin is nearly dead, get to the aid station, leave the Garmin on, plug it in to charge it, toss all that junk in your pack, and put on the cheap watch so you know what's going on for the next hour and a half. After an hour or so, your watch is recharged and will finish out the race, and you'll have a single complete data file for a hundred miler.
My particular Garmin doesn't look like it's recording data while it's charging, but I'm told this works. I'll definitely give it a whirl. Thanks for the tip, Marshall!