There may only be 5-20 calories in pre-workout, especially if it is a sugar free type. Some purists may sniff at that still, but I doubt that it is enough to spike insulin or stop ketosis at all.
Personally, I take creatine, pre-workout, multi-vitamin and sometimes low sodium V8 (for electrolytes) during my own type of 20-24 hours fasts prior to an hour long work out. I have not had trouble maintaining or adjusting weight because of this or felt anything strange.