Most likely Opus doesn´t recognize text files by itself, but because it has a well defined list of what text files are (doc, wri, rtf, etc.). In other words, i don´t think
you can do that by normal means, unless the programmers include a function to open any unknown file as a text file. You can use the preview pane on many file
formats, though.
Rather than tweaking the registry you might want to consider creating some buttons to open the selected files (whatever file type they may be) in Notepad or whatever program you want.
For example my primary text editor is NoteTab Pro. When I left click the button (see screen grab below) NoteTab opens normally. If I middle or right click the button then Opus sends the selected files to NoteTab. The advantage of doing it this way is you get to decide what program you want any particular file type to be opened in. I have similar buttons set up for sending selected files to the Photoshop, UltraEdit, FastStone and others.
Opus looks at the actual file contents to determine whether or not an unknown filetype is text or binary data.
If the file is binary data, opening it in Notepad is just going to show you a load of garbage in most cases. (A hex editor might make sense, but Notepad itself does a very poor job of displaying non-text files.)
Note that the Opus viewer pane will show you a hex dump in this situation (or if it's a file containing text, it'll show you something similar to what Notepad would).
But, having said that, if you do really want to do it, modify the Double-Click action on the Unrecognised File Types item at the top of Settings -> File Types.