I've re-read it and I don't read it like that - there is a sense that you could log on as SA, and then RESTORE (which I was not about to adopt as a strategy!!) , but I didn't read using EXECUTE AS from it. I wish I had done though!
What would be nice is if I could determine, FROM a backup file, WHO the original owner was and then, if that user exists on the target server, dynamically set the EXECUTE AS
All our backups are made under our control, so I could include the Owner in the Comment within the Backup, and then parse that out during the Restore.
(I don't restore using an SProc, but I do use an SProc to prepare the RESTORE command. I can give it as much / little info as I have; for example, if I give it a parameter for the database name it will list for me all available backup files, along with suitable EXEC SProc syntax which then includes parameters for database name and filename, so I can then cut & paste one of the listed filenames into the SProc command. Once I have Database Name and File Name I get an Sproc syntax for a full set of commands - which includes target folder (system default is offered) for MDF/LDF. Finally, once the SProc has "sufficient" parameters it spews out the full RESTORE syntax, including options for a DIFF and TLog backups, commented out bits to set the DB back to READ_WRITE, the code to rescue Orphaned Logons (if Login doesn't exist it includes a Create for that, and a commented out command for a Drop User in case, as part of restoring the DB, I want to get rid of stale users)
Long story short, I could include the EXECUTE AS in the generated code, based on something parsed from the Backup Comment.