iPhone with external hard drive I see there have recently been several products released that offer to give an iOS device access to local mass storage on the order of terabytes via WiFi.  It appears that most if not all of these devices only allow access to the drive through a special (and very limited) iOS app for THAT product.

Via jailbreaking or otherwise, is there a way to use ANY iOS app (for example, photo, audio, and video editing apps) with local Wifi-connected storage?
No, it is not possible in iOS and I'll bet it won't be implemented in near future. To be this feature possible, you need at least very fast wifi everywhere.