You need to go into powershell or the disk management console and mount the SD card as the dropbox folder.

I didn't see any posts about that here, but found a few articles that might be helpful:

http://winsupersite.com/article/windows8/surface-tip-microsd-content-libraries-metro-apps-144658

http://answers.microsoft.com/en-us/surface/forum/surfwinrt-surfdevice/mounting-a-micro-sd-card/8e371761-5c9a-4801-a782-d34e7732da5f 

Thanks - how do you do that?

See previous post.

You could also use an NTFS directory junction, I have a few set up for Steam games, etc.

See http://www.sevenforums.com/tutorials/278262-mklink-create-use-links-windows.html