I believe Jake is asking how to add labels to the airfields on the in-flight map, which would mean editing the image file.I don't believe this is possible. RAF_Louverts maps are a good choice when displayed on a second monitor
Lou's maps are great indeed - I frequently load them on a tablet next to the computer when flying.
In terms of the in-flight map, I think you're correct RJW - have now rummaged through some more files - adding labels etc. to the in-flight map would require adding info. to the actual image file. There is a "mapdata" xml file, however, available in OBDWW1, that the OP might want to look into - it seems that extra calls can be added in that file to sprites, data files and so on - so it may be worth investigating.
I currently fly in my FrankenWOFF install with Xjouve's black-and-white Michelin map, and a modified UI.xml file - so I don't have any further advice that may help (except to look into the mapdata file too).
(FrankenWOFF is a little side-hobby to load PE files into UE ver. 4.18 - to give SweetFX ability but with the improved files, aircraft, and other things, of the PE addon - it also makes the Fokker D.VI flyable in ver. 4.18, a ver. that never came with the D.VI - but I will not bore you with the details.)