First cool for Community, but im' not sure all people will need
MMO style character customisation

?
Specially for space, race or casual arcade games !
Technicaly, Oblivion have different type of heads you choose and the head is simply mounted "glued" on the body !
For the head customisation it has bones or vertex (i don't know) that are customized by the user !
Bioware have a bunch of AAA skilled programmers for these type of tool !
For 3DGS engine, i htink you need a simple and fast (not power expensive system) !
The best would be a simple visual base tool to attach anything to any bone of the model having bones you choose with possibility to adjust distance and orientation.
This way you could attach : head, hair, armor parts etc ....
You'll need to create your own file format with script to laod (and perform attach code),save !
Perhaps a simple file text having something like :
------------------------------
-main:Body.MDL
-head.MDL;0,0,0,45,0,0
-hair.dml;0,0,0,45,0,0
-sword.dml;0,10,0,0,0,0
....
------------------------------
The numbers being : offset x,y,z and rotation offset : x,y,z
Perhaps dig in this ??
character customisation example Good luck !