Got it fixed, I’m wondering if something is missing from the installer?
I added to HKEY_LOCAL_MACHINESOFTWAREClasses a new key called vmnt64.exe, and subkeys:
vmnt64.exe
+= shell
+= open
+= command
The default value for that last key is set to “C:Program Files (x86)WinCDEmuvmnt64.exe” “%1”
Then, I changed the default value for HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionExplorerFileExts.isoUserChoice to “vmnt64.exe”. (Somehow my account was explicitly denied write access to that key, but fortunately I could remove that policy)
I think the important thing was that first step, and that the rest could have been done using Open With => choose default program