I was actually going to work on a project that did something similar. Doing anything with large files while you're on a mobile device is difficult because of bandwidth, processing, and screen real estate.
If you could move or process large files though a third party without having to physically download the file to a temporary location, it would be awesome... but apparently Dropbox doesn't think so. I deemed my project DOA because I had a feeling that what happend to Boxopus would happen to me across multiple platforms that I wanted to integrate.