Woohoo! HTC 8X is back up and running! (Windows 8.1 Update, same version as before).
The download was complete when I got home, but it had not automatically installed, I had to click "Install" or something in the recovery tool.
I was concerned because it said to make sure your battery was at least 25% charged otherwise flashing might fail and make phone unusable - I didn't think I'd been able to charge the phone sufficiently when it was bricked because either the red charging light wouldn't come on, or if drained sufficiently enough to get to it, it would automatically go into the gears reboot-loop after 20 minutes or so of charging and the light would go off.
As it turns out I was okay. I had previously put it into bootloader mode and connected to my computer via USB (no red light though), then after 24hrs, unplugged and shut the phone off by holding the camera key down for 5 secs (if it was still plugged in it would have rebooted).
When I discovered the update today, obviously I was plugged in. I was out for 1hr20, it took about 15 minutes to install, then about 15 minutes to recover my backup. When it finally rebooted, I saw my phone was at 46% charge.
Because I'd backed up the phone before the W10TP update, I was able to restore the backup and only had to enter some email passwords to get back to how the phone was. Well, actually, it seems all of the apps have to be re-downloaded (many tiles are dark, meaning its app is no longer installed). There's an extremely long list of "Pending" apps in the Downloads section of Store.
Thanks HTC, thanks Gabe. Not happy having had to wait a week and a half with an unusable phone, but I'm thankful the phone was recoverable, better late than never!