I figured it out to the point where at least my problem is now solved.
Restarting in Safe Mode left userdata.omd editable in notepad. Restarting normally and then disabling all my startup items in task manager left it editable. Going through the process of elimination by selectively disabling startup items eventually led me to f.lux:
Disabling only that left userdata.omd editable, and with it enabled, killing f.lux with task manager also left it editable. Exiting f.lux with the context menu of its tray icon wasn't enough - it left a thread running for something called noshow.exe - but then killing that with task manager made it editable.
All pretty odd, but now you should be able to try it yourself if you want.