Can I create an ID mask for an object of multiple pieces? I have the rim, wheel, and treads joined together (using ctrl+J) for unwrap. When I use this technique, all the object are one color.

For that, you could paint a ID mask manually. Here's how to do it quickly:

• In Texture Paint mode, turn on Face Selection Masking in the header
• Select a face and hit Ctrl+L to select everything attached to that face
• Switch to the Fill brush with a strength of 1 and fill the selection
• Repeat with a different color for each piece.