You cannot just mash together the DC devs and expect them to work in a new fused manner. And even worse, you cannot just kidnap devs from other projects and force them work on nullDC. (tried that already, didn't work) XD
They have their projects in which they're comfortable and/or interested in and that's it. At least, that's how I see it.
Even if all sources were available, checking a particular solution in one to be implemented in another project can be quite hard if not impossible. That is, due to style and algorithm differences. There's a section in the nullDC wiki to help in that point: nullDC Wiki - Bugfixes, without having to get lost inside alien coding.
For example, you go check how MMU works in Makaron to port it to your project. So after getting comfortable with Makaron coding style you finally grasp what its MMU part is doing... then... you realize it was pointless since you might had already known how MMU works on the Dreamcast to begin with. After that, you also realize that your "core" ain't MMU friendly at all and would have to be rewritten. Yay! For now knowing that you have to make a new emulator, basically.
If you want to document the hardware, go check MAME then. If I remember correctly, they were kinda into that.
About implementations being always useful however... go try it. And you might find out that what works for one, might not even work for another. If you reach the point of understanding that particular implementation to begin with.
Why you still insist that nullDC's MMU support is a matter of looking at someone else's code puzzles me still. The problem is not knowing, is doing it. Those who can do it already have the knowledge, yet do not have the time and/or will to do so.
If you're so eager to see progress, then how about helping out yourself? It would only require some basic coding skills to get you started.
The code and the documentation is on googlecode. As for setting up the dev enviroment, is just installing VS Express and some SDKs. It's free, and quite easy. The most time consuming part is just downloading the stuff.
Anyhow, I don't see how this old discussion is of any use and/or related to the thread at all. I suggest you just get your hands dirty at reversing what you do not want (lovely thing about open source) and make a new thread if necessary.