Pre rendering avatar body and shirt spritesheets separately

Hi,

I want to give my users ability to create customizable avatars for my upcoming isometric board game. I am going to make a basic 3d model for my avatar and place 8 cameras and pre render its walking animation on a sprite sheet from 8 directions.

Now lets say i have designed a shirt on the body of this 3d model . Now the way customization is going to work in the game is that the shirt-only spritesheet will be overlayed on top of the 3d model sprite sheet so that the avatar will seem to walk with the shirt on.

Now my Q is, is it possible to prerender only the shirt on the sprite sheet of its own ?
If yes i think i will have to make the inner mesh for the shirt too(and any other accessory for the avatar for that matter) ? Is there a better way than this ?

Another question is the shirt casts a shadow on the persons body too. How do i preserve this shadow in the shirt-only sprite sheet ? so that when it is overlayed, it will cast a proper shadow on anything underneath it?

Also I am not an artist, any advice is much appreciated !
Thanks!

Just apply a mate material to the body and render the t-shirt together with the body.

What dTb wrote should work but you should make sure that the body is similar and has the same animations in all characters.
Here is a good tutorial on how to use matte shadow material.

Hope this helps.