The only way I could see of getting around the use of a Harley insignia on a piece custom leather work would be to have the customer buy the item in question, supply it to you, and ask you to incorporate it into your leather work. You aren't getting any money for the Harley item, you aren't getting any markup in any way. Harley could sue you of course. In America, you can sue anyone for just about anything. It's highly probable, though, that you could get that lawsuit thrown out.