After some Googling I found this: Use a Visual Studio Macro to Insert Copyright Headers into Source Files. It looked promising:
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After losing work to some obscure PowerShell script (not the one added as an answer), I decided to create copyright.py. Example usage:
C:\>python copyright.py "C:\users\me\documents\path\to\vsproject"
It recursively finds all *.cs files in the specified directory and prefixes them with the copyright text. It does not matter if other text already exists at the start of the file; it will be removed.
Note: As a precaution, you should always backup your code before executing this script