With the Directory Security Feature of DocumentsCorePack for MS CRM 2011, you can prevent users to see certain files from CRM records. Users will not be able to navigate to the associated directory within the file share or SharePoint because DocumentsCorePack will overtake the security for you. In short words: when a user is not able to see the CRM record, he/she will also not be able to see the documents.
For a detailed description, please have a look at the documentation.
However, you have to do additional steps for the Directory Security.
In an IFD Environment it is hard to implement a FileShare which is available through the Internet. We found a product named WebDrive where you could configure, for example an FTP which will be mapped to a local drive.
Here is a link to the product website: http://www.southrivertech.com/products/webdrive/winindex.html