Yes, you're right, those won't work, but this should--try setting the min-max value for the trial as "0-6000" in your style file. Then, for your time, try "rt:any,full"

The full keyword tells DirectRT to maintain the current stimulus for the max value even if the response occurs earlier. If you use "0-6000!" then it will also end the trial at 6000ms if they have not yet responded.