Well, it did not work. I prepared a full Monterey (latest version) boot disk on an external SSD and the MBA 9.1 boots from it fine but no firmware update runs. Looking online Monterey seems fussy about not having an Apple-branded boot disk when trying to update firmware and some people have forced it using teminal to load/run the correct firmware file from the OS installer. Seems like a lot of hassle so I'll just be happy with 526.0.0.0.0 for the time being.

