It seems like that is no longer the case. I was able to successfully retrieve a non-standard variable using the `UEFIv2` PowerShell module [1] (which is just a thin wrapper around the undocumented `NtEnumerateSystemEnvironmentValuesEx` function) without actually naming the variable in question.
To the untrained layman like me, this sounds like Windows actually is querying via `GetNextVariableName`, because UEFI doesn't seem to offer any other interfaces that aren't "get/set variable by name".