in a purely deontologic moral system [..] there is a list of "bad" things, and the system is very simple: You may do thing X if and only if X is not on the list.

Are you asserting that purely deontologic systems don't include good things which it is preferable to do than leave undone, or that it is mandatory to do, but only bad things which it is mandatory to refrain from doing?

And are you asserting that purely deontologic systems don't allow for (or include) any mechanism for trading off among things on the list? For example, if a moral system M has on its list of "bad" things both speaking when Ganto enters my tent and not-speaking when Ganto enters my tent, and Ganto enters my tent, then either M has nothing to say about whether speaking is better than not-speaking, or M is not a purely deontologic system?

If you're making either or both of those assertions, I'd be interested in your grounds for them.