I'm sorry but I don't really understand what you are trying to do. Are you using a UIA only writable volume? IF so you won't be able to safe a file inthere because you are not using the profile directory in the writable. Only way to succeed with that is to let UEM create the file for you. Because UEM executes within the user context there is no need to use variables.
I am using UIA Only writable volumes and from what I see in the snapvol.cfg file is that it starts out with 'virtualize=\', virtualizing the entire c:\ drive. Then later it adds 'exclude_path=\Users' in order to remove user specific settings from the Writable drive.
I want to add back a single file from 'C:\Users\<UserName>' into what the Writable volume captures.
This file is created by another application and holds some user config info that disappears on the whole 'refresh on logoff' procedure.
EDIT: I have a workaround of copying this file to a redirected Documents folder on log off, and bringing it back on logon, but would still rather have it saved in the Writable volume.
For as far as I know this is not possible. It is either everything or nothing. You could off course include the root and then exclude all and every file from it but I'm afraid it will still see it as part of the profile and thus load it like that. And because you excluded all other folders you will end up with a new profile every time you log in.
This is just hypothetically, you would need to test if this is true, or ask support if it is possible. My guess is it wont be.