Yeah, the fact it's M2 makes me lean towards it being (extremely good) emulation as well, with their internal emulator just having lots and lots of hooks to do all the gadgets around the sides. It's just the way they tend to do things, they want to emulate the hardware slowdown as accurately as possible.
I did speak about how it could be using some deep internal emulation.
I figured these "hooks" you mentioned have triggers embedded into it that fundamentally change how the original code works.
So it wouldn't just be as simple as extracting a "ROM" from the code base (would also require cleanup).
As stated a few times I haven't looked at it... IF I can be bothered to grab the NSP I'll unpack it and have a look around.
I'm no Cave fan, so my motivation to do this is beyond low/I have other projects going that I actually care about.
Also full disclosure I'm no "coder" my talents/skills come from a place of piracy (I've learned just enough to steal shit and when I dump games for myself/the internet its because I don't want to pay for them).