Sounds very interesting, but writing a how to on that topic, one that could be useful for you, may be very tricky. I'm afraid that pre-smartphones were not as homogeneous as nowadays phones. Still today there are differences between the terminals from different manufacturers but before it was even more.
If you haven't found any details on how to access those files via software, then indeed you can open the thingy at your own risk and take a look, but be ready to destroy the device. There are many chances it will not work again, specially if you do not have the right tools... Also be ready to probably see a single board with a lot of chips difficult to identify and many of them under some metal cover or epoxy. If you are lucky:
- You may found the memory chip (that is very unlikely) and to get the part number to find its datasheet and figure out which pins you have to work with to read the memory.
- You may found a Serial Port for servicing. This usually have contact pines on the board instead of a connector. Usually there is no serigraphy saying... "Hey!, this is the service serial port"
- You may find a JTAG port to use standard hardware debugging tools
I'm a complete n00b myself on this topic, specially because, mostly at the beginning, it requires ridiculous amounts of time before you start to see advances.... and also it becomes expensive when you break things down while trying.
Good luck and hope you share what do you find in this adventure!!!