Nice topic idea, I don't believe we had a custom bosses topic yet.
Have a look at DELTA; when I eventually get all that done it will indeed be capable of much more than was originally envisioned. It is what came from me trying to create DKCLB, and is now the project that will make DKCLB work (DKCLB is an extraction toolkit now, and it will run/work
within DELTA Suite).
Custom bosses will entirely be possible, since DELTA is going to be a generic game engine capable of far more than just DKC.
Try the latest DELTA demo
here if you have no idea what I'm talking about.