You said somewhere it is not production ready, is that in relation to engine or editor?
That's primarily aimed towards the editor, most of the low-level C++ should be fairly stable but there could still be issues in less used systems. Although in large majority of cases the issues are superficial and fixed quickly.
Is it possibile to replace systems with custom ones?
Certainly! Most systems are implemented as dynamic libraries that implement the interface provided by the engine, and this includes physics. You can write your own dynamic library that calls Bullet instead of PhysX. Take a look at BansheePhysX project.
You also mention not wanting to use the component system, which is also possible. All relevant components come with non-component variants using a more traditional API, just remove the "C" prefix from the class name (e.g. CCamera for component, or Camera for non-component version).
I understand it is being ported to other systems and currently it is only for windows?
Currently only Windows, but Linux port is currently being worked on and essentially done/working, macOS to follow right after. Mobile platforms and consoles will come a bit later.
Is the asset pipeline flexible with regards to modding? Is it possibile to load raw files during runtime without converting them to binary format?
Yes. You just need to make sure to load the raw file import plugins during start-up (take a look at start-up code for the GettingStarted example) and you can call the raw file importer any time.
How many people do work on it and is it planned to be supported for foreseeable future?
Just me and an occasional contributor. There are many features planned for the future, with official release coming soon and a few projects close to my heart being built on it. I'm not planning on stopping support any time soon.
Is there any community around it yet? IRC?
There is little community around the project at the moment, which I mostly contribute to my stellar social and marketing skills. These forums and Gitter chat is all there is, but I'm pretty quick to respond to both if there are further questions.