How can you preview 512 x 512 textures if you don't have those items?
Do you use some sort of third party previewer?

No you just use tga files.
Find somone who has 512x512 textures,go to his custom folder take those texture and copy them to your custom folder.
After that you can just put images on tga files.

But what about forces and stuff?
Take the item.dat file from their custom?

Convert the texture files into .tga using photoshop and GIMP's save as feature and put them in "tori" folder.

Run the game and the textures will be shown.

Also, here is a detailed guide on item.dat editing:

http://forum.toribash.com/showthread.php?t=195093