ok, i'm not a c# coder, but went digging and found this in Updater.cs
baseCommand = UrlVariable.Replace(baseCommand, "file", "\"" + job.PreviousLocation + "\"");
so it appears as if that field is going into the {file} variable as you say, but it isn't being displayed in the client?
i did find a section where the comments indicated it was faking a download to fill custom variables.. but since the previous location is stored in a local db and is referring to a localfile, shouldn't it be possible to extract this on application startup? in theory it could be displayed when completely offline..
no matter, i'm sure you know your code better than I