I've never tried custmizing but I do know a little about obitsu's
they are suppose to be easy to take apart thats how they are built you can pop the arms and legs out easily as well as the hands http://junkyspot.com/obindex.html see here they are going to squeak its that new doll sound when I got my 27cm boy he squeaked in some areas now he still squeaks in the hips but the rest of him is squeakless
I'm not sure by what you mean by the shoulder joint is stiff how stiff can you still move it? again I chalk this up to "new doll" and once you play with her a bit her arms will be more manageable I advice against using oil since it could eaither eat at the plastic or make her arms/shoulders too lose and fall out easier when you don't want them too or not pose well enough the reason they are so "stiff" is so she can pose well here is a guide on obitsus http://www.youtube.com/watch?annotation ... feature=iv