I currently got a PHP/jQuery library that I've been working and improving for the last 2 years, and I'm using it in most of my own projects with great success. I'm always working on it, adapting to my needs, and then I decided to make it open source for other people to use, I'd like to ask for feedback but not sure if it's frowned upon on Stack Overflow, since it's full of willing developers with all kind of ideas, paradigms, etc. Could I do it? Or where else would it be viable?
You should probably ask on Code Review. From their FAQ
HOWEVER, if you dump several thousand lines of code into a single question then I doubt it'll remain open for long. Equally if you just link to your project then this will be closed as it's useless for the future.
What would be okay is to take a single module, or better, part of a module. Formulate this into a working piece of code and then ask about that.
There must be specific parts of your code base that you're more unsure of than others? If you're looking for constructive feedback this would be the place to start.