Unrelated: I've been thinking for awhile that it would be nice to have plugins that can override the default components (note list, editor, viewer, etc.). I know others have been thinking about this as well.
My thoughts on this stem from the JSheets plugin which would benefit from overriding the editor and the viewor, so that when you switch to a spreadsheet note, you just get a spreadsheet. This system would also allow for alternative editors (maybe), and alternative renderers (like ReStructuredText or maybe even latex?).
This system would also allow for a very simple pin lock plugin. It would be able to override all components until a pin is entered (I won't comment on the security of a plugin like this, or my own view on it's validity).
Actually, I don't know much about how dialogs work in Joplin, it might already be possible to open a (nearly) full screen dialog that won't close without a pin.
Paying for a feature is different from paying for the development of a feature. If you do have the means to pay for development of a feature, I suggest you look into contracting someone to make a private build for you. The Joplin code base is open source (MIT License) so your contractor will have full access. As you said this is a "simple feature" so they shouldn't have trouble with it.