I have successfully achieved a way to have large files on the external, and it was quite simple honestly.
I only tested this on a 160GB drive, but I assume it works on any drive.
This is what I did....
1. Took a 160GB hdd and hooked it up internally.
2. Allowed the PS3 to install it's proprietary files, format, fix file system.
3. Booted to hack, filled up the hdd with games.
4. Put the drive in an external case and hooked it up via USB.
5. It works.
I did this 2 days ago and looked around to see if anyone else already did this. I found nothing.
NZHawk's Awesome MountPoint Manager v1.0 used with this method is great. It isn't hard to figure out what else to do. Naysayers will want proof, I'm not looking for recognition. I am just sharing what I discovered. I was curious, tried it and it works.
I do have a question though.... How hard would it be to make a homebrew app that would install the PS3's file system on an external without having to go through this process?