I just saw this thread, but I guess I don't understand. MC can generally access files stored on any filesystem that can be seen by the OS, including UNC paths and drive letters on Windows, and anything you can mount on OSX.
So... What exactly do you need MC to support? Dropbox can certainly be accessed on the filesystem.
Not if the files are not synced to your pc (meaning that they are only stored in the cloud and not locally, which is the ideal way to store lots of data in the cloud), that's when you will need Netdrive or a similar utility.
I've been playing around with this in the past few weeks. Actually, that's the reason why I tested MC in the first place: I first tried to get iTunes play files stored in MS OneDrive (where I have unlimited storage, thanks to my Office 365 subscription), but I wasn't 100% satisfied with how iTunes handled it. Mp3's stored in the cloud would play fine, but it took ages (sometimes 5 seconds or more) before the next track would start to play.
MC handles files in the cloud (not synced locally), including FLAC. a LOT better. It's possible to get gapless playback from Google Drive and Hubic (a lesser know cloud provider). OneDrive and Dropbox were much more hit and miss, but in general playing hi res files from the cloud is just not good enough yet. Some days I could play FLACs for an hour without any hiccups or pauses between tracks and the next day tracks would keep buffering, skip to the next track without warning, etc. In the end I decided to keep storing my files locally for the time being. I will stick with MC though!