Alright, I have a really ugly solution to this problem that satisfies the requirements... Or at least I think I do. My initial testing seems to point towards it working the way it's supposed to, but be warned that this is likely well into unsupported territory so if anyone reads this and wants to use it know you're probably on your own. This method will also cause the Dashboard to throw a warning about more specific routes every single time you go to commit a change. It can safely be ignored, but it will do it Every. Single. Time. 😞
First, set up a default route on the hub MX, with a next hop that doesn't exist, and set it to be active when the host responds to ping. By doing this the route will never be active, but it will add a 0.0.0.0/0 option in the Site-to-site VPN page that you can include in the VPN.
You should have something like the following:
Here's the VPN page with the never-active quad zero as an option:
Now that we have the correct option present we can configure the other end of the tunnel to match the 0.0.0.0/0 encryption domain, and voila! Full tunnel from non-Meraki spoke to Meraki Hub.