but if the server running your reverse proxy (i.e., Caddy) is connected to your Tailnet that has the address 100.x.y.z, then it should, in theory, be able to reverse proxy to that address.

But Caddy won't be able to give it a certificate... because the global DNS is resolving to an IP that isn't reachable over the internet.

This idea isn't horrible, nor useless. Using Tailscale to act as your internal network (i.e., it's the network that your machines talk to each other on) is neither far-fetched nor pointless.

I'm all about using something like tailscale as the VPN layer between a VPS and your home network, to host things on the internet without port forwarding but again, this configuration won't work...