Solution with processing module didn't work since processing module works with admin page only, and login is required for admin pages.
I'm working on simple module for user register/login/logout, very basic one, and I wanted to have common url for all module related stuff. Like at least: www.example.com/user (not to mention /user/login, /user/register, /user/logout). I don't really like the idea of placing business logic in template files, and I would like to avoid it.
At the moment I'm going back to where i started and i will prepare some in-between solution - I will create a hidden page (accessed by /user/), and I will hook to on before render, and check if I'm on that page, and then process any proper requests, and finally I'll do redirection (so /user/ page won't be rendered).
And I could really form question otherwise since I also wanted to know how to connect module with specific url (so it can be used as form action for post requests).