Inspired by bsag's applescript, I've created one that works nicely with quicksilver and my new api method for tracks. I'm going to attach the applescript here. Here are the instructions:

1. Download the AppleScript.
1. Edit the script to include your own username, token, context id, and url
1. Compile and save the script
1. Copy the script to ~/Library/Application Support/Quicksilver/Actions/ (you may need to create the Actions directory)
1. Restart Quicksilver
1. Activate Quicksilver (Ctrl+Space by default)
1. Press "." to put quicksilver into text mode
1. Type the description of the next action you want to add
1. Press tab to switch to the action pane.
1. By typing or scrolling, choose the "Add to Tracks" action.

That's it! To be google-friendly, I'm going to include the source of the applescript here, too.

using terms from application "Quicksilver"
	on process text ThisClipping
		
		(* You need to edit these lines with your own info *)
		set myUsername to "YOUR_USERNAME"
		set myToken to "YOUR_TOKEN"
		set myContextID to YOUR_CONTEXT_ID
		(* Almost done with stuff to edit. Change the url on the next line, too. *)
		
		tell application "http://YOUR_DOMAIN/backend/api"
			set returnValue to call xmlrpc {method name:"NewTodo", parameters:{myUsername, myToken, myContextID, ThisClipping}}
		end tell
		tell application "Quicksilver"
			show notification "Tracks: action added."
		end tell
	end process text
end using terms from

Happy GTDing!