SMEStorage with Dropbox on the iPad
SMEStorage is a cloud storage service, similar to DropBox itself. In the scenario you are referring to, you can use the SME Storage API directly to upload to an external file provider like DropBox.

Effectively like DropBox you can store everything on the SMEStorage Servers and then access it from any OS/Device as long as you have an internet connection. 

From reading the documentation it is really a more advanced DropBox service, which allows developers to use their API to have a central point for accessing various storage options. 

From what I can determine, like iCloud, the application needs to support the actual API and interface to be able to store data through it, and since iWorks on iOS only supports iCloud, I am not sure if it will solve your problem.