Can you use a pipe?
Edit lpx_read_model to use standard input and pipe the information to it.
You might play games with the FILE struct.
Edit lpx_read_model to edit the FILE struct after it opens a file,
any file that will open for input.
I think that you would only need to change three entries, but I'm not sure.
If lpx_read_model does all its own IO,
you could just provide it with alternative functions that use your string.
E.g. globally replace fscanf with fscanf_str.
You might get the same effect with #defines.