Thank you for your replies, everybody.
Of course, I'm NOT looking to have the extension .epu associated via Windows Explorer or the Windows registry with ebook-viewer.exe; that's both inappropriate and unnecessary.
All I was hoping for is that if ebook-viewer.exe encounters a standard Windows/DOS short filename in the 8.3 format
as a command-line argument when ebook-viewer.exe is being called, which, as Windows itself will style it, has the .epu extension, that ebook-viewer.exe would recognize it instead of rejecting it. No part of calibre would ever try to steal the file association from any present or future program that used the .epu extension.
Thanks!