Edit: Assume the device is not jailbroken
Here's what I've done so far: I have looked at libimobiledevice and ifuse. From what I could tell, they were basically developed for Linux, but they are supposed to work on OSX. I was able to compile them for OSX by using Homebrew, but I have not been able to figure out how to actually use ifuse as it takes a mount path, and OSX doesn't seem to expose the device's mount path.
Has anyone actually been able to access the Documents directory on the iPad/iPhone via the OSX terminal window? How did you do it?
I think of this as needing a simple host for an http get. What's worked well for me is to place the file on volatile host like FileTea, and add a text field in my app where you type the url for a get.