When I left my last full-time job to return to freelancing I walked in on a Monday morning and gave my two weeks notice to the boss man. I went in with the goal of trying not to piss them off and in the process turn them into a client/customer. Similar to what you're angling for from what I can tell by your comments.
So two tips:
- Make sure you're ready. I knew going in, just like you feel you do, that they would be open to my plan. A lot of the time leading up to giving my notice was spent making sure I was right. I quite literally spent three months getting the balls to jump ship and making sure I was pretty confident I would come out with a client.
- For the love of God, play it cool when you tell them. Try and remember that you're quitting a job and not making a sales call. I wouldn't lead with your plans until they ask. During our conversation about the transition, the chit-chat slowly worked towards what they should do when I was gone. That's when I threw out that they could hire someone else full-time or take me on as a contractor for the technically heavy work and dish out the rest to existing employees.
The only thing I find worrisome about your scenario is that you're apparently counting on them buying a product that doesn't appear to exist yet. They can say "Sure, great idea!" on the day you leave and then, months down the line when it's time to play ball they can act dumb and say "No thanks, baby. That was just pillow talk."
I feel like you might be putting the cart before the horse a bit here but I wish you the best of luck.