There should be no problems mounting a hadlebar bag with exposed derailleur cables. People have been doing this for decades before somebody calculated the drag produced by cables when touring at 30+ mph.
Save yourself some time and money. Buy the bag and mount it. You can always opt to change to brifters, if this does not prove to be satisfactory. N.B. finding a suitable mounting for a handlebar bag may be a problem because the bike is equipped with front pannier racks and not a handlebar bag support. However, this has nothing to do with the presence or absence of brake and derailleur cables.