Does anyone have a suggestion on just how they would implement them?
Reason devices MUST be @760 pixels wide, half rack effects aside. VSTs come in a massive variety of all shapes and sizes, so you definitely can't put them in as a rack device.
So then, do you have a plugin "holder" device, sort of like a Combinator, which can handle it's ins and outs, maybe some mapped controls, and when you need to actually edit/operate the plugin itself it would then open up in a floating window, like in other DAWs?
A huge part of what makes Reason cool is the freeform routing and modulation possibilities. None of those would apply to 3rd party plugins. They don't speak the proprietary CV "language" that is in Reason, so you wouldn't really be able to integrate them with any other Reason devices besides audio effects. Props would then have to build some kind of CV to MIDI translation so that you could really integrate them. Also, VSTs have no standard for the number of audio and MIDI in/outs they have. It varies case by case. How would Reason account for that in a rack device?
Then you get into the aesthetics of it. Reason very obviously adheres to a distinct look and feel. The moment you add 3rd party plugs that all goes out the window. It's much more important to the usability than people realize. Hell, it IS the usability.
There are a million more things to consider once you start really thinking about it beyond "I want plugins in Reason". It might not be that great of an idea.