I see little value in this. Remote avatars are not actually copied to the board server, so the basic assumption ("feature already there for avatars") is faulty.
The sole benefit I see is that huge files might be added to a post much quicker by using the bigger bandwidth of servers. In this situation (admin offering big files as attachment), a MOD should be quity easy to program.
For other use cases (i.e. small files), it's perfectly possible to enter an URL in the file selection dialog in most modern operating systems.
No support via PM.
Trust me, I'm a doctor.