What is the effort required to use a game engine such as Unreal or Unity, etc. and create an avatar customization features...complete with clothes.

The user should be able to customize the body features and the clothes need to then fit onto the customized body.

What is needed? Can you create one set of 3D models for clothes and somehow programatically have the clothes adapt to the body shape? I.e. The same shirt model will be able to fit on a skinny person vs. someone with a big beer belly.

How difficult is this? What are the steps needed to implement this avatar creation/dressing feature.

I'm basically talking about something like in Rockband 3.