The ideal GBB flags for this setup seem to be 0x11. That enables fast booting but doesn't force legacy devices; there's no need for dev_boot_legacy or dev_boot_usb. You can destroy and recreate the Arch partition as many times as you'd like. It's running pretty well for me so far - Arch is a lot faster than Chrome OS and I haven't run into any problems yet despite the old kernel. I don't know of any way to get a linux-headers package for this, though, so you're out of luck if you want to compile modules. You can install other kernels (or the default kernel), but there's no way to boot them.