You may want to re-phrase your question, the post doesnt seem to make too much sense.
From what I gather you want to make a CP out of MDF with a swapable metal center section. You said that you want players 2+3 to be the swapable ones, I assume you mean the two player stations in the center? These two posistions tend to be players one and two. Player three tends to be on the left and player four is normally on the right.
What exactly is it you are having problems with? The construction theroy, the patch panel or......?
Anyway, I like the idea of this!
My understanding is that he wants
|.--------------------------------------|
| P1 | P2 | P3 | P4 |
| Fixed| Mod- | Mod- | Fixed |
| | Ular | ular | |
|____.|______|______|______|
Like that, and it seems his questions are in regards to everything, both how to design it to work and how to do all the hookups.
Now if thats the question my suggestion would be to make the whole panel out of metal (the top atleast) and then the center spot modular, this should give you a more seamless (clean looking) design.
As for modular panels, the keys here are labelling and lots and lots of planning.
You need to plan out what modular jack (in the link the person used a patch panel, which is quite clever IMO) you need to make note of what jack correspeonds to what button on your encoder (which in this case will likely be one of the larger controllers like the IPAC
4Remember the better you label and plan out you can potentially avoid any reconfiguring of your emu program when switching panel pieces.
As for the theory behind the link. What he did was (as the best I cna tell) he wired a seperate jack (read, patch panel) to a different input on his encoder. Somewhere he has a list denoting what # = what key input. So when he is swapping panels he knows that "ok, for this game I need 4 buttons (say A, B, C, D for example) and one 4-way joystick and a start button so he gets out those panels and plus the attached ethernat cables into thier corresponding jacks on the panel and he is ready to go.
Something to keep in mind is that the person's cab we are reffering to did a ONE player panel, the more stuff you add the more complicated it can get. So major planning is a must, I really cant say that enough

Hopefully that begins to answer your questions.
{Footnote:}I would like to say that I have never built a panel or a cabinet, by experiences comes from that of wiring large buildings and infrastructure solutions that are involved with my job as an IT consultant. Including the general knowledge of basic electronics and connections. And what I have learned from reading many projects and browsing this site for the last few days. Basically what Im saying is I can help you with wiring and theory but the layout of your panel is beyond me
