What we're forgetting here is that JK Labs was not a taper or even a recording geek, yet his products work great and according to the users sound nearly identical to the mic bodies. The box that the cables plug into is very basic and there's nothing secret about it, it was probably built from a schematic he found somewhere. I doubt that loads of research went into the building or refining end of the products so in that sense the results could be replicated fairly easily. I'm just trying to open the door here, this knowledge hasn't really been discussed before and I couldn't find any real info about active cables anywhere. Just like anything else, it can be done by the right people.
Any idea of how different the systems would be from MG to Schoeps to AKG to Neumann?
Jon may not have been a taper or recording geek, but that doesn't mean he isn't an audiophile or doesn't know his stuff. I believe he did sonar engineering on a Norwegian sub. At any rate, I've had a number of interchanges with him and have also gained alot from the posts he used to make on this board. I think you are way, way off base to think that he did not spend alot of time on doing designs, using his ears, along with his training, to discover what worked best. And he certainly did not just build stuff from a schematic he found somewhere.
My AKG ECMS box is the ECMS-22. I talked to him about why he named it this and it turns out it is the ECMS-22 since it was his 22nd revision of his ECMS design (Extended Capsule Microphone System). From what I gather from my interchange with him, he tried lots and lots of different parts in the design to find what sounds best. BTW, to my ears, my ECMS-22 does not sound like my 480 bodies. I think it has better clarity and detail, but it is definitely not the exact same sound as the 480s. (I would like to do a good comparison to the JWilliams mod 460s at some point.)
I'm not sure what this all says about reverse engineering, but I do think it is quite reasonable for someone who puts this much time and development cost into making a design for market to price that product in a way that captures this R&D effort (ie, not just priced on the cost of the parts used in the design).
And all this is not to say that your friend (or someone else) cannot build a good mic active system. But it will probably be much more than just finding a schematic somewhere. It's going to require prototyping and testing out different combinations of different parts to find what sounds best.