With a tailscale mash network setup between remote machines, ad-hoc remote builds can be achieved with:

nix build --eval-store auto --store ssh-ng://[hostname] ...

This will not copy the build results into your local store. Use nix copy-closure to copy the build result from the remote store to your local store.

nix-copy-closure --from [hostname] /nix/store/...

Pass the –json flag to the nix build command to get the output paths of the derivation.