Ok, thanks. I've found the solution I was looking for here:
For anyone interested, now I don't block the shell extension context menu handler with ShellExView but add its CLSID to Directory Opus blacklist in Preferences - Miscellaneous - Advanced, in the ignore_context_menus setting. This way it still shows up in a Windows File Explorer context menu as before, but doesn't show up in its original form in a Directory Opus context menu, neither in a simple one nor in a extended one, but my custom context menu which uses a command to reference by label the original one works as intended. That's the exact behavior I wanted in the first place.