This is a great idea for a plugin, I just came across it as I was using s2Member for the first time and wanted to upload member files.
It works well for PDFs or similar, where you upload them and insert them straight into the post from the Secure File Uploader popup. It gets a bit trickier with images though, as one would often want to align them or chose a specific size.
The problem occurs with the Add Media function, where the secure files are showing in the gallery (attached to the file), but the file paths are wrong - the system thinks they're in the normal uploads folder.
Is there any way to flag the secured files as just that, in the system, so that the file paths are correct when trying to insert something via Add Media? I seem to remember BuddyPress had something similar, with protected file uploads..
Would be very interesting to hear your thoughts on this!