We've had requests for this feature for years. I can't stress how much customers request this feature; it's put a lot of egg on our face that Dropbox beat us to it.
In order to do this on Mac, we'd need to register ourselves as an accessibility client. I don't remember the details about registering ourselves, but from what I remember, it doesn't require hacking into OSX.
We've had to hack into OSX in the past: Adding menu items and icons to Windows Explorer is supported via well-documented Microsoft APIs. It wasn't until about 2014 that Apple supported this, prior to that, we had to reverse-engineer Finder. We didn't get OSX APIs to do this until we hired a contractor with "connections" to Apple he petitioned his connections to provide an API. I know that Dropbox, Google Drive, Box, and an open-source project called Liferay-Nativity all performed the same hack.
Based on my Syncplicity experience is that, what happens in these cases, is that a product manager gets so focused on the pixels that he/she is completely blind to the practical implementations. There's probably a bit of "I told you so" coming from some of Dropbox's engineers now.