Has anybody created a (open source) mod of info_intermission that allows the mapper to point to a text file and make it print the contents on level change? Or one that achieves a similar effect to Quake's end of episode texts but without having to hardcode the text? 

You could make use of the trigger_boss_gate.

This are the logic endings of an episode and switch with one of the four end runes to the text file, as they are used to complete the cycle.

Only thing you also have to do is change the progs.dat with a texteditor and use the same amount of text and replace it with your own.