thats very different though. those add-ons are pretty much free for anyone to use, but lets say someone had a private add-on that they didnt want to release and now everyone who joins their server has it.
Yeah, I know. Downloading add-ons straight from the server is just stupid. Even if someone did make this, you wouldn't be able to send anything other than text files; no images, sounds, music, models, etc.
I have an idea for private add-ons
If it finds anything in the text files that references "private", non-case sensitive, it will not download.
FTFY. Still not a good solution. When it comes to making your data public, you always want to assume that the default option is to NOT share a file, and only have a select few that you would want to share (aka a
whitelist system). What you've suggested is the opposite; you assume the file is public unless the author went
out of their way (something not many gamers do) to note that it's private (a
blacklist system).
I want to find add-ons without spamming S&R
I suggest you improve your ability to find stuff on your own. This is a great tool:

If you can't find what you're looking for with that, begin a google search with
site:forum.blockland.us. So, for example:
site:forum.blockland.us development tools