It could be done, but you'd need to change the scripts at least a bit so that when the game is loaded it combines the plots from multiple files into one in-memory dictionary/javascript-object.
You can also just split a big plot line into smaller sub-plots using a jump node (eval_jump_to). From the player's perspective this will look exactly the same, but the editor will not evaluate a jump node and simply end drawing the graph if it encounters a jump node.
I currently have a game with 19 384 nodes (the plot.txt is 3.3MB) and the size of the plot is not a problem. True, some paths are convoluted and terrible to view in the editor, but there's a 'Draw from this node' button that hides pars of the graph that are unreachable from the select node.