Well, that is a pickle.. That's unfortunate. I will preface this by saying I'm not responsible for you causing further damage, but you could try finding a screw removal tool/backout bit, or maybe a pair of needle nose pliers.... However, that thing is so small you might not be able to get a grip on it.. Try at your own risk. You also would need to be VERY careful to not touch anything else OR get metal shavings on anything.. That would be nearly impossible I think. You may cause further damage.. Make sure you do a backup of all your data somehow before you try anything!.. Time Machine etc.. If this was the first time you've taken the back off any computer and worked on one, I would suggest instead, finding a reputable local Mac repair shop. Good luck.