Well, that was a great big pile of weird. The first AP, I was able to just download the ME firmware to my laptop, run an TFTP server on that, and use the IP address for the laptop's wireless connection in the ap-type mobility-express tftp://xxx.xxx...
. The AP happily pulled the image off the laptop and converted itself without issue. Hooray I thought, that was easy.
When I came to do the second one, I factory reset it in the same way as the first, connected it up in the same way, but it just didn't seem to be able to connect to the TFTP server on the laptop. I tried deactivating the Windows firewall, but there wasn't even any evidence the AP was trying to download the file, it kept complaining about timeouts or the file not being there. I could ping the IP for the laptop, but the TFTP just didn't seem to work.
That's when I started to think it was something to do with the existing firmware version on the AP and I started trying to downgrade the lightweight firmware version to the one Paladin mentioned, and of course then I found I was unable to access those versions from Cisco, hence my post just above.
Anyway, in a fit of exasperation / desperation / genius I turned on the TFTP server on my TrueNAS box, stuffed the ME images on there and issued the conversion command one more time, exactly as I had before, just changing the IP address to that of the TrueNAS box.
And whaddya know, it grabs the file and converts itself with no bother.
I got them both talking to each other with one as controller and the other as a subordinate AP, now I just need to work out how to do wireless bridging / "mesh".