Running Tailscale without privileges is a challenge because tailscaled needs to be able to configure your network, and if you enable Tailscale SSH it also needs to be able to create sessions for configured users. For people who dont need SSH and accept this challenge + maintenance burden, it is possible: https://tailscale.com/kb/1279/security-node-hardening/
If you're excited about tailnet lock and want to get on the alpha sooner rather than later, feel free to drop me an email. As Dave mentioned we are slowly crunching through the waitlist to get some miles in, but I'm also happy to take on enthusiastic testers ahead of that!
You could also use tailnet lock in this fashion, by issuing a `tailscale lock sign` command for the new node once you've verified the provenance of the new device. Because it involves signatures with keys on your device it could never be as simple as a REST API, but maybe we could offer a more easy to automate command or better client library support (suggestions welcome!)
Thanks for the feedback!! Writing the documentation for how this worked was a challenge, and its good to hear what pieces we need to call out more strongly in the future.