Not super familiar with fly.io, but with a quick look at that page it should work just fine.
Just instead of dropping that camellia.conf to the WireGuard MacOS client or Linux wg-quick, spin up the TailGuard container somewhere (pretty much anywhere, but with good ping to fly.io). That way you should have the fly.io private network accessible in your Tailscale tailnet, it runs wg-quick internally alongside Tailscale anyway, just with a bit of scripting to automatically configure the network and the firewall to avoid connections leaking.
If it doesn't work, feel free to raise an issue and I can have a look.
Just instead of dropping that camellia.conf to the WireGuard MacOS client or Linux wg-quick, spin up the TailGuard container somewhere (pretty much anywhere, but with good ping to fly.io). That way you should have the fly.io private network accessible in your Tailscale tailnet, it runs wg-quick internally alongside Tailscale anyway, just with a bit of scripting to automatically configure the network and the firewall to avoid connections leaking.
If it doesn't work, feel free to raise an issue and I can have a look.