Well, it's been a considerable while since I originally posted this and nobody has actually answered the question.
However, I can report that I had similar problems with my current persistent USB install this evening. So I had another go at recovering my personal files from the USB stick and the solution is actually mind numbingly simple....Rather like I can be sometimes it would seem!
I really think you're gonna kick yourselves when you see this...I certainly did when I thought of it.
All I needed to do was:
1. Create a folder in the /media directory
e.g.
sudo mkdir /media/jas_casperfs
2. Mount the casper-rw file on the USB stick in the newly created folder:
e.g.
sudo mount -o loop /media/USBSTICK/casper-rw /media/jas_casperfs
{simultaneously rolls-eyes, facepalms and slaps head!}
Once the casper-rw file is mounted, you have access to the entire virtual drive and all of your documents!
Why didn't I think of trying that all of those months ago?? Grr!
It just suddenly occurred to me, I knew all along that Linux can mount and read most of the major filesystems from the last god-knows how long. But for some reason it never occurred to me before that casper itself is a filesystem. I never made that connection before, but as soon as I did, I figured it must be supported, so it should be possible to mount the casper-rw file like any other filesystem...
And it bloody works....Unbelieveable!
Anyway, my long journey is now over and anybody else who experiences the need to retrieve files from a persistent USB install using casper-rw will also know how to retrieve their files!
So this is finally and definitively solved!