I wanted to make some default replacements for the clothes in CS. Is there any way to get them to show up in Bodyshop?

I doubt it. You can try to make custom versions and see if they show up.

I don't know how to get Castaway Stories content into Bodyshop, but you can take stuff that you made in Bodyshop and use it in CS. You need to copy the file SavedSims from; My Documents\EA Games\The Sims 2 and paste it under; My Documents\Electronic Arts\The Sims™ Castaway Stories. Most of the custom faces, heads, skins, cloths, jewelry, etc. will be available in create a sim. The cloths will be in the shops. The hair, makeup, etc. will be available in the mirrors. If you want to use parts that you downloaded and modified then you need to copy the Downloads folder from; My Documents\EA Games\The Sims 2, also. If you get bugs during game play after copying the Downloads folder, then go through the copy of the Downloads folder and delete the files that are about objects and changes to game play. Be sure that "catalogs display custom content" under game options is set to on. That worked for me.