I dunno if this is all necessarily relevant, but d3v used the DanAdam-signal, so here I am.
You’ll probably have a problem with using a KOC turntable in a from-scratch build.
My experience with them lies in USKOC modification, but both work the same, you have two offset optical sensors and teeth on the PCB, and a rotary encoder (quadrature encoder) is used to determine if it’s moving in a direction. Now this may be different on a JKOC, but on a USKOC there’s no place to access that “pulse” of CW or CCW movement, as that’s contained inside the chip that talks to the PS2. The JKOC PCB has way less self-contained logic, so it’s possible the pulse exists in there somewhere. As much as I love modding I never really probed that PCB, shame on me. There’s a few other ways to feed the turntable input into a PC only having those sensors as your turntable output, but it gets complicated from there, and won’t work (outright) with Trilogy.
As for 8 buttons with a KOC, this isn’t ideal but you could try adding on a foot pedal (it’s its own button on a KOC, mapped to a turntable input in IIDX) and using the turntable as one button and then 1-7 as the others, with the footpedal as Fever. You could even I guess forgo the turntable and just press Start or Select for Fever. Other than that, yes you’re right, you need 8 buttons to play 8 button mode.
Now, disregarding any KOC modding, from-scratch without a turntable involved isn’t that bad. Here’s the buttons they use in IIDX machines:
Expensive, I know. I’m a bastard and got 14 of them for way too cheap.
You can approximate those buttons with HAPP poker buttons, but they will need to be mounted differently to make sure they don’t wiggle in place. The IIDX buttons mount into a box-shaped hole and have an inner bounding box on the inside that squeezes the button into place with a nut below it (kinda hard to explain). Whereas the poker buttons mount with a circular hole (same size as a HAPP button I think) and the entire “box” area of them is on top of the surface you mount them to, instead of mostly below like IIDX buttons. A friend who used poker buttons to make a IIDX controller used a router (the wood-cutting kind, not the internet kind lol) to make a recessed square hole that lets them sit at IIDX height and also keeps them from wiggling. IMO if you don’t want to bother with recessing them (though buttons sticking way out of your controller seems weird to me), you could use some sort of adhesive to hold them in place. I guess a very tightly nut will hold them decently, but if it loosens and your buttons go crooked, it seems like it’d be a pain to realign them over and over.
As for modding a Muse-On, I don’t know much about that. I believe I’ve heard that you can’t just lift out the old keys and put new ones in, nothing as simple as that. I think you can use Sanwa springs for IIDX keys + Omron switches for IIDX keys and give it more of a precise feel like a Sanwa IIDX key. I suppose with tons of dremel work (it’s a whole crapload of dremeling if it’s anything like adding Sanwa IIDX keys to a IIDX KOC) you could fit Sanwa IIDX keys in there directly.
Edit: looking at that disassembly link, I don’t think there’s enough height to directly fit Sanwa keys, disregarding any other issues of fitment. If you go this route, you could knock out the area underneath the keys so they stick out of the controller, then do something to raise the controller up (for my Sanwa-modded KOC I’m going to mount 1" thick wood under it and knock out a hole under the keys in the wood).
To this I say, “Uh… You realize Tech Talk is basically a modding forum…?”