@pjz99: You're generally correct, but the process isn't that hard.

To make a hybrid you need new neck geometry, or at least a modified neck.
The head and the body meshes stay exacly the same.

If you look very closely at the picture I posted, you see a small discoloration around the middle of the neck.
It's because the head and the upper part of the neck of my "prototype" still use V4 textures while the body and lower part of the neck use V3 textures.

So, if I could use a V4 head that is already remapped for V3,
only a very small part of the (new) neck would actually have to be remapped.
The larger parts and the body and the head wouldn't have to be modified at all.

The far bigger problem is that the new neck needs custom made morphs so it matches with the default bodymorphs.

But with Cage and Spanki's morph transfer script this has become easier to do than ever.

Of course the original cr2 has to be modified so that it combines V4 head morphs with V3 body injection channels, and some joint parameters for the eyes and head will have to be changed a bit, too.