You'd have to fully exit Opus (via File / Exit Directory Opus) and re-run it after renaming it for the rename to make a difference. But also note that running a renamed dopus.exe will cause other problems, so this is only good for a quick test.
Making a copy of notepad.exe and calling it explorer.exe is an easier test.
But it does look like Perforce are intentionally hiding their menus from everything except Explorer, since they aren't appearing in Notepad (when named Notepad.exe at least) and based on the thread Jon linked to, so you will probabvly need to complain to the Perforce team and ask them what they were thinking when they made that design choice.
It's possible they have a configuration setting to control where the menus appear, as is the case with some of the Tortoise source control tools.