function [varargout] = read_write_json (options_filename, options_dir, rw_mode, varargin) %% This function read input parameters from a json file and translates to a Matlab structure, or % ...

Of course once I post this I had a kind of lightbulb moment. When writing a new file just write a List<> of a single item, and then the serilizer creates the JSON array and not a single item.