I had an “Aha!” moment—OK, so it was more like an “Oh, duh! Of course” moment. I’d cloned a World Adventure accessory with morphs and alpha transparency to make the kimono jackets, so I assumed you would need WA in order for it to work in-game. Then I remembered reading before that in order to make EP items base-game compatible you have to change the OBJD/CASP Group ID to 0x00000000. This seems to have solved two problems I was having—first, making it base-game compatible and second, getting the thumbnails to show up without all the hassle. Who knew it was as simple as changing that one number? Who knew?! No, really, which one of you knew about this and didn’t tell me? I kid ;-p
The Accessory Kimono Jackets are now base-game compatible. Be sure to delete the old files before installing the updated ones.