This new product from onOne PhotoVia – ON1, Inc. (former www.mosaicarchive.com Mosaic product) could be a near future option for you. They mention Dropbox as one of the cloud storage option. Having used Mosaic in the past, the product was able to sync automatically from LR and the folder hierarchy btw your computer and the cloud storage was maintained.
If you want to mirror the folder structure in your catalog with Dropbox, but you don't want to publish your entire structure, can use the Folder Publisher plugin: Jeffrey Friedl's Blog » Jeffrey’s “Folder Publisher” Lightroom Plugin. The author has an excellent reputation earned from his many plugins. You can publish just the photos you want to a folder hierarchy in a Dropbox folder.