Am currently working on a project, and though I can add sidebar items by adding to the com.apple.sidebarlists.plist file then restarting Finder, I can\'t seem to know how to
There's no supported way to do this using pure Cocoa. Use the LSSharedFileList API, instead. It's the supported way to manage the sidebar lists, and you can provide a custom icon (as an IconRef from Icon Services) when you insert an item.