Not really, just push the tarball with sources to nginx when you deploy a specific commit.
You can wait for the first person to request the source and do it then manually (or link to the nginx github).
Most likely that will never happen for most people who deploy it.
After all it's a pretty good price for what you are getting in exchange.
I fail to see what is unrealistic about that, or indeed user-unfriendly, unless you consider the user to be the developer and not the person on the other end of the wire.