If I'm correctly understanding what you want to do, it shouldn't be hard. Remember that submit files are evaluated lazily, so the PRE script of a node can write the submit file for that node.
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There are a few more details, like removing real_submit if it already exists, but that's the basic idea. (To summarize, the PRE script can create the submit file however it wants to -- DAGMan doesn't actually read the submit file until it's time to submit the job.)
Thanks Kent, I'll try something like that! Cheers, Szabolcs