Thanks MrC!
I can confirm the menu doesn't seem to do anything in Opus.
However, I then tried it via Notepad's File Open dialog, which is a good way to test if a context menu literally only works in Explorer. Trying the Media Center > Play option on an MP3 file via Notepad's File Open dialog results in Notepad crashing every time for me.
So I'd say there is a bug in the JRiver Media Center context menu shell extension, and the bug is causing different effects in different programs. They are likely making an assumption about the host process which is only true within Explorer.exe.
You'll need to report this to JRiver for them to investigate, which will hopefully be easier for them to do now they (probably) only have to look at the problem in Notepad.exe and not Opus.