You'll be able to get an initial set of data from federation but you won't get any data beyond that.
Federation works via "pushes", but since your instance would be behind a VPN the other instances in the federation wouldn't be able to see it to push content updates.
Tailscale has funnels, you could check them to see if they can forward the traffic to your instance.
I don't know exactly how it'll work since the funnel has a specific URL provided by tailscale, so I don't know if a cname or something like that could work for a custom domain to your lemmy instance