"Fake"/Wizard of Oz checkout

Hi! Thanks for reading this thread.

To validate a startup idea, I would like to create a fake check-out process to run a so-called Wizard of Oz Test (or Smoke Test or whatever you would like to call it!).

I would like people to put stuff in their carts, go to the checkout and stop them as late as possible with a message "Sorry! We're not ready yet, but sign-up for launch discounts." 

What I've done so far is nice but not good enough. I have set up the commerce side of things, removed the cart icon from the top bar and replaced it with a button saying "check-out". That leads to a page where people are notified that this is just a test. What's actually missing on that page is the cart content and that makes the test not good enough.

Does anyone here have any thoughts on whether it's possible to push this experiment a bit further? Is there anyway to change the links behind buttons in the checkout? Or does anyone have any other suggestions? 

Thanks a lot! And yes, the person that comes up with the best idea gets these launch vouchers!