Texmod could be used to make notes on the map, but you would not be able to add or change those notes while in game. A good example would be marking a safe running route, or concise directions for speed clears.
I think it would be more optimal to get a fully mapped picture on a different program where you can add notes/routes etc.
Then you can just open that on one of your other monitors, or alt/tab it which isnt that much different then going into map view all the time.