If you make modifications, and deploy them in a way accessible to a given user, the changes must be made available to that user.
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.
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.