Are there existing open source projects that accomplish this?

Yeah I was going to ask the same thing. It seems very likely there was some OS option you could have used here.

There is an OAuth1 module[1] that is available, but not only is that not the version of OAuth we built in our provider, but liboauth conflicts with certain packages our servers need installed.

If there were an OS option, I would have used it :)

[1] https://github.com/yaoweibin/nginx_http_oauth_module