Rather than try to fuck with the traces in the keyboard (irritating - only time I've ever successfully messed with that involved painter's tape and tin foil), I would either:

Solder the first solderable point from each button contact on the DMG board directly to other buttons mounted on the keyboard somewhere else - maybe you could remove an un-needed set of f-keys, mount a piece of wood or plastic there, and drill to mount some buttons - or just drill in the big open space at the top of the keyboard

-or-

cut away from the keyboard case at the bottom there, cut the front of a DMG in half and put the original buttons back in

Whoo, thanks for the help arfink! That answers pretty much all of my questions. I think I'll just hit up one of the EE Majors to help me figure out how the pot connections work.

If it works, I will be so ridiculously happy.

@TraceKaiser
The problem is that if you look at the board, the Start and Select buttons are covered by the case, and the bottom of the D-Pad just clips the case as well. Larger Picture
So in the interest of looking nice and uniform, I'm going to try and cover the entire area around the screen, and then just wire the keyboard buttons to the contacts on the DMG PCB. Although, now that I think about it, there might be a better way to do this, like putting the A and B buttons on the side, Start and Select through the case at the position they are now, and then mapping the D-Pad to the arrow keys like planned before. Hmmm, I'll have to play around with it I suppose. This entire project was just started on a whim really so it's just one grand experiment. ^_^

I guess the biggest issue is just figuring out how to place everything inside the case. I have significantly less room than expected, but still enough to barely squeeze everything in. On to rewiring everything ever and hoping for the best!

I've gotta say, this is a fucking awesome idea!
You don't need to rewire the buttons though do you? Cause the LSDJ keyboard lets you use the d pad and buttons doesn't it?
Don't have one, so I'm not too sure.

If you can sort out the cart slot, this will be amazing.

Ok, lemme try and answer some questions:

Yes, with careful soldering, you can relocate just about anything on that PCB. You can. It'll be fine.

For wiring the DMG buttons/contacts onto the keyboard, be careful. Most keyboards have plastic membranes inside which will be literally impossible to solder to- the plastic will melt at far lower temps than the solder. So what you'd need to do is map out the keyboards matrix for both the top membrane layer and the bottom one. Then, you'll know what pins connect to what keys. Of course, you'll have to isolate the signals some way- something tells me the DMG and the keyboard's motherboard won't play nicely together if you don't.

Removing the DMG speaker has no adverse effect.

You may wire all the DMG's ground connections to the keyboard's ground plane if you want, but it's not strictly necessary IMO.

Adding extra outputs to your DMG will not adversely effect it unless you decide to connect all three of them up to something simultaneously. I that case I'd be worried.

So I've successfully made an LSDJ keyboard. But I want to be able to play the thing without having to deal with clipping a DMG to my belt and potentially entagling myself in wires. Solution? Put the DMG innards in the keyboard casing and make it a Keytar!

(This isn't wired yet. I just cut out the Numpad and placed the screen PCB inside to see how it looked...)

So this is pretty much the first electronics project I've ever done, so I've got a few questions about wiring and all. If anyone has any tips at all, it would be greatly appreciated. First of all, I am going to have to wire a new volume pot so that I can access it through the keyboard casing. However, I am horrible at deciphering the DMG Circuit Schematics. So I think I've figured out that I will need a 1K ohm stereo pot to replace the current dial thingy, but I'm not sure where to solder what on the PCB. If anyone has help for this it would be great. (PCB/Volume Pot Picture) I will have to do the same with the contrast dial, but as it only has 3 connections, it should be easier to figure it out on my own. (Although I gladly welcome any knowledge you have.)

Also, can I remove the cartridge reader component and place it somewhere else in my casing with ribbon wire to the PCB? Is this even a good idea? This is critical, because if I can't do this, I won't have any room for that back PCB due to it's thickness.

And now for the less detailed questions:

How can I wire buttons on the keyboard to the contacts for the DMG buttons? Is it as simple as carefully scraping away the top layer and soldering? Or is there something else I need to do?

If I remove the DMG speaker, will it affect any of the circuitry in a way that should concern me?

Can I just wire the all the ground connections on the DMG to the ground plate on the keyboard?

Does soldering two or three prosound mods to the PCB affect the sound quality or limit the DMG's ability to even run? I would like to have 3.5mm stereo female out, 1/4" mono female out (stereo to mono conversion done internally), and if I have room, RCA females out.

Whoo, that's a lot I don't know. I also might try underclocking it for good measure since it is supposed to be a performance instrument. I'll probably have questions about that as well later. Apologies for my lack of knowledge, but I'd appreciate any help you guys could offer.