Thank you for the follow-up!
I found out that the technique I originally wanted to implement was described in the thread above. I know that’s a breeze in .xib, but the current filter plugin protocol is allowed to have only one dedicated menu item per plugin, so my assumption is that we can’t have an alternate menu item or multiple keyboard shortcuts (with modifier variations) unless we duplicate the plugin. It is technically possible to manipulate the main menu after the plugin has been loaded, but I would avoid that to keep my plugin concise.
I know filters are not meant to provide a menu item that behaves like Edit > Paste Special, so the current situation is completely okay for me, and my NSEvent-based hack is working nicely so far. Otherwise I should go implement a proper dialog!