Two questions:
1. Can I ftp a file directly to my dropbox folder?2. Can clients access that file directly via a link embedded in my (proprietary) software?
1) Dropbox doesnt support FTP
2) Yes. However, note that often link formats are changed and so may suddenly stop working. Same if they hit bandwidth limits.
Simply put Dropbox is not a content distribution system/network and shouldnt be used or relied upon as one.
