I dont think you can change the GET READY text but you can change the TEXT when the MISSIOn and INTRO loads.
EG: It usually says something like Date, Time etc and i think GET READY (
) Instead of the GET READY sign u can have anythign u want using "OnLoadIntro" and "OnLoadMission", these have to be put in the INIT.sqs file or the Descr.EXT file (cant remember which)
You can also disable the date and time, cant remember the command for it sorry.. Something like ShowTime = 0 (Or False)
Hope this helps.
Rhys