There might be a shorter way, and I might be misunderstanding the problem, but...
Install SB Settings with Cydia
Install Open SSH - both the SB package and the stand-alone package
Install WinSCP if you're on PC... there's a native way to SSH with Macs, methinks... Find instructions, its
pretty easy. You'll need to have your iPhone WiFi address at hand. Settings-General-About. In WinSCP, Login : root. Password : alpine. Port : 22 Host : wifi address of the device. iPhone must be on wifi and not docked; OpenSSH in SBSettings must be green for the rest of the procedure. Turn it off again to red when you're all done.
Google the path for the iPod Cache photo thingy...
Find it with the SSH protocol provided with WinSCP
Copy the videos in any file you want. Don't drag, use the Copy/Paste feature only. Do not move any file in WinSCP. If your internet connexion isn't much fast, copy them one by one.
When you're done, you may remain jailbroken if you want, in which case I recommend changing your SSH password to something else than alpine. Or, if you were stock and wish to return to that state, once you're sure SSH is off again, remove the Open SSH packages with Cydia, and then you can just do a backup with iTunes, and then Restore officially with iTunes as a new iPhone, and then Restore from backup (right click on your device in the left tab in iTunes, select latest backup) to get all your things back - except the jailbreak.
I'm not quite sure what the iPod cache thingy is, but I'm pretty sure you can get anything out with SSH. In any way, now that I've answered, my guess is someone's gonna come up soon with a better solution... or to confirm this one.