There’s a simple editing plug-in available for this task called Borders & Titles and as the name says you can add simple borders and text to your images within Aperture 2.1. It’s a sample plug-in provided by Apple and it’s fully functional. You can download it from Apple’s Developer Connection website without registration:
The downlaod package includes the source code too so you can adjust the plug-in if you’re able to code Cocoa. After downloading put the BordersAndTitles.ApertureEdit plug-in into /Library/Application Support/Aperture/Plug-Ins and access the plug-in from Aperture.