Are there any ingredients I should not add to a no-knead dough with an extended proofing time? I am having great results with no-knead bread, generally leaving this to proof for up to 18 hours in a coolish (20 C) room.

I'm thinking of upping my game and adding fresh ingredients (e.g. minced fresh garlic, salami, tomato etc.) to the dough. Is there any ingredients I should avoid for food safety or quality reasons due to this long proofing time? 

I will be using white bread flour and approximately 1/4 Tsp of bread machine yeast, and possibly sugar and olive oil as well.
I don't think there's anything specific to worry about with bread dough.  The important point is the temperature: if you're considering adding an ingredient that you normally wouldn't let sit out at room temperature for 18 hours for food safety reasons (and/or because it might start tasting "off"), you probably should add that ingredient toward the end by folding it in before shaping or something.  

Most no-knead bread recipes that sit out at room temperature for a long time are quite flexible.  You likely won't cause any problems by folding in an ingredient late in the first rise.