I've been meaning to write it up, but just haven't found the time. I finished the BMW and now I'm swapping an '07 Vortec 5.3 into my Jeep. Nonetheless, here are the highlights:
Its a little more involved than just repacing the D-Jet ECU. You need a new harness and all of the D-Jet sensors really need to be replaced as well, but its not really that hard. Megasquirt and most aftermarket ECUs generally need the following to operate:
IAT - intake air temp sensor
CTS - coolant temp sensor
TPS - throttle position sensor
MAP - manifold pressure sensor (usually in conjunction with or in place of an air flow meter)
AFM - air flow meter (not necessary on Megasquirt and some GM-based systems)
O2 sensor - lets the ECU trim the mixture within limited parameters
Idle bypass valve - let's in more air when cold - can either be a simple switch (like L-Jet) or controlled by the ECU for full idle control under all situations
Ignition trigger -- some way to produce a signal that the ECU will recognize
Those are your major components. It is extremely easy to add the IAT, CTS, MAP sensors to you stock D-Jet manifold. O2 sensor just requires a bung welded into one of the exhaust downpipes. The TPS is a little tricky, but not a big deal once you find the right one. I found one from a Ford Mustang that is compatible with GM and Megasquirt. You may even be able to use the stock auxillary air valve thingy under the manifold. Then your left with figuring out how to trigger the ECU and mounting up the injectors.
There are numerous ways to trigger the ignition and the ECU. Most ECUs use either a Hall sensor or a VR sensor. My goal was the keep the motor looking as stock as possible, so I used an L-Jet distributor with the stock VR sensor. You can run that straight into most aftermarket ECUs and they'll recognize the signal, then send out a signal to trigger the coil or an MSD. Some ECUs may require converting the signal through a GM ignition module first, but that's no big deal. You can either let the distributor handle the timing, or convert it over to full timing control. I did full timing control. I disabled the advance in the distributor by removing all the guts and epoxying the two pieces of the shaft together.
Then you'll need to figure out your injectors. I have a Megasquirt I built a few years ago, but I didn't actually use it on my coupe. Instead I went with a GM ECU intended for a tuned-port V8 and reprogrammed it to run the six cylinder. This required changing out all of the D-Jet low impedance injectors for some high impedance ones that are compatible with the GM ECU. The problem is that the typical Bosch-style high impedance injectors have a different fuel rail mount that require custom machined caps or some other unique method to properly mount them. In hindsight, that was a lot of trouble to go through.
I believe Megasquirt is actually configurable to run the D-Jet low impedance injectors without frying the injector drivers. You can either run external resistors, or set it up with a programmed solution -- its been a while since I looked into it, but I believe they called it pulse width modulation. Check out the Megasquirt site for more info. So in theory you should be able to reuse your stock D-Jet injectors, fuel rail and regulator with the Megasquirt. I haven't tried it, but wouldn't hesitate to do so.
The final big item is a wiring harness. You can't really reuse the D-Jet harness since none of the connectors are compatible. I carefully removed mine and stored it away with all of the other stock D-Jet parts (I figure they may be worth something some day). Then I made up a new harness using wiring and connectors from here:
http://www.rs-autosport.com/
The wires were only long enough to reach under the dash. If I had to do it over, I would get extra long wires and run it all the way back to the stock location.
This isn't intended to dissuade you from upgrading the injection. It is absolutely worth all of the time and effort. Plus, this is easier in most respects than a motronic conversion using the D-Jet manifold. I think the only part that is more difficult is the harness, but Megasquirt makes that relatively easy, too. I believe there are a couple of people on the board who are actively investigating/installing a Megasquirt. Hopefully they'll chime in with an update -- or tell you what I missed or got wrong.