Grimrock

Thank you so much Grimwold, I was afraid that I would need a script entity after all. I would have liked to be able to have a single piece of code that expanded the spell library of any dungeon without having to have to edit that dungeon, neat and clean, but this is as good as it gets. I was considering spawning a script entity with OnCast that is pre-defined in the script-file so that you can avoid placing a script marker in the editor. Would that be a workaround?

This could be possible... the framework created by JKos makes use of externally created scripts, and 3socks provided a good tutorial of how to reference external script files here:
viewtopic.php?f=14&t=4013&hilit=external#p41932

It makes use of a trick by placing an element in the dungeon, say a door that opens at the beginning of the game, and when it does so, it creates the script entity. I have not tested it myself though.