Some background on how it's set up:
The main site is using .NET MVC and, as you know, the forum software is using .NET web forms. I was able to get a universal site login by switching the default Microsoft OWIN Identity login to use the YAF OWIN Identity login. This still lets all of the OWIN features persist on the rest of the site. The user can register or log in from either the main site or the forum. They can use their same account to comment / upload on any page of my site and also discuss on the forum. Additionally, it is using YAF's External Login Auth for Google and Facebook with a couple tweaks, such as allowing the user to create their own username.
Check it out!
Big thanks to the_watcha for all of the help and support throughout this process!