Ah, well, the solution to both of your plug-in questions is... Alpha channels! In order to get the transparency slider, you have to have alpha channels in your images. If the alphas are in place, the transparency control will be automatically created. And as for removing black outside of, but not inside your object, again, alpha channels. Add Pin is for luminescence effects (light sources); for 3d objects of any kind, you want to use alpha channels. If you are creating them in a 3d program, there is a good chance that when you render you can render the images with alpha channels already in place. If not, here is a link
that explains how in Photoshop, and here is a link
explaining the process in Pain Shop Pro. There is also a simplified tutorial on creating this type of plugin right here
I'm not sure on the deleting plugins thing though. I'll have to see what I can find.