Fully interchangable models

I'd like some thoughts on how games such as EQ and Dungeon Siege allow so many options in your personal characters model. I can see a few changes can be made by using multiple textures and simply swapping them out, but others require whole new mesh chunks.
I was thinking perhaps I make the initial player model merely a biped and perhaps a pelvis with numerous mounts. After creation, I mount the chest, the head, the arms, etc. All of which have their own mount points for further items like weapons and partial armor. Full armor would remove the currently loaded meshes, and replace them with thicker, armor looking ones.
Does this sound about right? Can I even load a partial mesh as the default player model to begin with?
Thanks,
Erik