Any app can access e.g. Dropbox directly, no? It'd just have to "display" it as a filesystem. Probably too much work..
You said "mount". I took you literally. (Got downvoted for it too; no idea why.)
What I had in mind was basically FUSE (like https://github.com/joe42/CloudFusion). I don't think it would violate sandboxing, but it's probably very non-trivial to implement in this kind of app