I've serviced the brain on my 2006 sworks stumpy several times over the past few years, mostly because of the crummy turnaround times from spesh, and the fact that it never makes it through more than half a season before losing lockout etc. And the fact that I'm a mechanical engineer and stupidly tear into anything that breaks thinking I can handle it (half the time I can't).
I have to admit it took me about a year to get good at rebuilding it, the critical thing is you have to make sure there are absolutely NO air bubbles in the system when you refill it. Air bubbles getting into the hydraulic fluid seems to be what makes them fail.
There are a few threads out there that are helpful, and there's a rockshox monarch repair video that's similar enough to give you some good ideas. There are also schematics available on the net that are a huge help and also tell you what size o-rings etc you'll need (and make sure you replace them every time, at the least the standard ones which are easy to get from the internet or in my case my friendly neighbourhood hydraulic component shop), they're cheap and if you don't you have a good chance of failure. Note that some of them are Vitron (teflon impregnated), unless you have absolutely no choice replace vitron with vitron of the same color (durometer).
I had the schematics saved somewhere but haven't found them yet, that's how I ended up here, I'll try to remember to check back once I find them, and if anyone wants let me know and I'll pass them on.
I don't bother with nitrogen, I think the main reason they use it is the convenience of a high pressure cylinder of nitrogen versus using a hand pump or a high pressure compressor (you want to charge the brain side to 350-400psi). Some shocks do just use air. I put a schraeder valve on my brain and use a suspension pump to fill it, I can get about 350 psi into it. If you wanted nitrogen you could probably go to a motorcycle service shop and get them to charge the system for you.
To refill it I have used Fox fluid, but it's just rebranded motorcycle fork fluid marked up about five times as much. I've used Belray motorcycle fork fluid with success (It's available in a variety of weights so you can experiment a bit, I think I used 7 wt). I've also tried transmission fluid at someones suggestion, it works fine but smells terrible, I doubt I'll use it again. Whatever you use you need a lot because you need to have a bath of it you can fully emerge the whole system in while you reassemble to make sure you don't get bubbles, for me I need about a gallon minimum.
There's probably no reason to take apart the shim stack, but if you do be very careful and methodical, take your time and take LOTs of pictures. If you dont get it back together exactly the way it was it probably won't work, and I've only found shim stack diagrams for one or two models of brain.