Would be easiest to use BackingStore

[linas]

I'm assuming you've lost interest in this, but .. just in case you ever plan to come back to it, I thought I'd leave some comments.

Instead of doing what you're doing here, I think there's an easier way, easier, at least, if you have access to C++ bindings to prolog. That would be to implement the get and set functions in this file: https://github.com/opencog/atomspace/blob/master/opencog/persist/api/BackingStore.h
That file has like a dozen calls in it, but only about six are really key; the others can be skipped.

There are two (actually five ...) examples of how do do this. The most readable examples are these two:
https://github.com/opencog/atomspace-rocks/tree/master/opencog/persist/monospace
and
https://github.com/opencog/atomspace-cog/tree/master/opencog/persist/cog-simple

Besides the above, I can offer general brainstorming services, e-mail me if you ever get interested again. I mean, there are also lots of other ways to do this, too...