I just wrote an AppleScript which allows you to caption successive images in Aperture. Check out:
http://www.tow.com/software/aperture-caption/
Basically, the script does the following:
1. Lets you enter the caption for the currently selected image
2. Calls up the Batch Change menu option
3. Pastes the text
4. Hits Enter for you
5. Moves to the next image
6. Repeats steps 1-5 until you hit Cancel
You never need to touch the mouse! With FastScripts Lite --
http://www.red-sweater.com/fastscripts/ -- you can even assign a keyboard shortcut to the script, making it really easy to caption images on the fly.
Enjoy!
-adam