Add .dropboxignore directory to exclude folders without using selective sync

An example where this is useful if not completely needed, is sharing a DropBox folder between Mac and Windows on the same machine.  I am not talking about having two instances of DropBox running--one in Windows, the other in Mac--pointing to the same folder as I have seen that completely mess up the dropbox folder with lost files and such.  I am talking about having one 'side' running the software and controlling all syncing with the service from its own Dropbox folder, then having the other side just access the folder directly as if it were any other folder shared between the Mac and Windows.  

The problem is since Mac and Windows store hidden/temp files differently, inevitably the client is syncing a bunch of things it shouldn't be.  In my case, my DropBox folder is now littered with files all starting with a period.

Specifically in my setup, I run OS X Yosemite as my primary OS so it has Dropbox set up syncing to a folder under my user's home directory.  I then run Parallels Desktop to run Windows, but thanks to shared folders, I can see my Mac user's home folders, and thus I can access the Dropbox folder from Windows.  Even when I make changes to the folder from the Windows side, the client on the Mac side, monitoring that same folder syncs all changes to the cloud.  Again, to be clear, the Windows side has no idea of anything called Dropbox. It's just accessing a shared folder from the Mac side.

The problem is because of this setup, my DropBox folder is now littered with almost double the number of files because for every actual file, there's a corresponding 'dummy' file with the same name and extension but with a leading period.   Here's an example...

.MyNotes.txt

.SomeFile.pdf

MyNotes.txt

SomeFile.pdf

The first two share the same filenames and extensions as the latter two but start with a period.

Since *I* know I'll never create a file that starts with a period, I want to exclude them from DropBox, but I can't, so instead I just have all these files littered throughout my DropBox store getting synced to *all* my machines and devices. Worse, since the file extensions are still the same, all clients think they are jpgs, pdfs, etc. and display them as bad data.

The only solution is to set up *two* dropbox stores, one on the Mac and one on Windows but that would mean I am literally doubling the storage space.